mikuni flatslide flatspot

i have fitted a single mikuni 40mm flatslide carb to my stock 750 commando i am using a k&n filter no balance pipe on the exhaust downpipes pea shooter silencers and lucas rita ignition my problem is i have a flatspot at 6500rpm if i hold the throttle fully open it will gradualy pickup and when it reaches 7000rpm it picks and will carry on revving to 8000rpm if i let it i only do this for a few seconds ,pete v has been a great help to me with this i have gone from a 170 main jet down to 145 i have tried the needle in every position and fitted and fitted a bigger needle jet ,is this a main jet problem? bearing in mind it will rev to almost 8000rpm on a 145 main jet! thanks for any help ....baz

Flat spots are LEAN spots... you do the arithmatic :wink:

Hi Baz, I had similar symptons just recently on my 850 Roadster with 2 x PWK JS Motorsport flat slides, it had a flat spot at 4900 to 5200 rpm if you gassed it hard and when cranked over heading up or down a winding hill after being cranked over either side, would falter when stood up.
If i increased the throttle very slowly it wouldn't do it.
Anyway turned out it they were way to rich, I went down 2 x jet sizes and got rid of the of the 2 x small wire style aircleaners that came supplied with those carbs from Jim as they were half the problem, just not big enough to let enough air in.
I now run 2 x UNI filters that came with a set of Premiers that I got here on the forum.
Regards Mike

40mm seems too large for a stock 750 with only a K&N filter and basically stock exhaust. Is there a reason you chose 40mm?

That being said, this sort of carb tuning problem could be difficult to sort out. I'm a little surprised that the sorting so far took you smaller in jet size because usually this type of problem is related to fuel starving as RPM's increase rather than too rich a condition.

Does it just have a flatspot without any other issues (backfiring, spitting...etc.)?

Have you checked the plugs for burn? Exhaust gas analysis?

If it were me, I would want more info on how the engine is running the rest of the time...lean, rich...that would help hone in on what direction to take.

I agree with Dennis in questioning your selection of such a large carb for a 750, or even for an 850, why?

by far the favored choice is a 34mm

even a single 32 can deliver enough gas in the air/fuel mixture

the issue is in flowing enough air, and not enough gas

when you attempt to properly jet a very large single carb like a 40 you have to make it rich enough to handle the increased air flow, or you have what you have, a too lean flat spot

why complicate unnecessarily the air/fuel mixture by selecting a 40?

The bike runs extremely well much better than with the 34mm vm mikuni that I removed the mid range is mach improved .the reason I used the 40mm flatslide was because I bought it to run on a drouin supercharger so I could do away with the fuel injector but this has been shelved for now until my 920 engine is built.So I thought I would try it ony 750 I started with a 165 main jet and went up to 170 with no improvement I also fitted a richer needle jet but this made no difference .after some consultation I went down on the main jet but still no difference. I did have this carb on a 34mm inlet manifold it is now on 40mm one but the flatspot remains between 6500 and 7000 rpm the plugs are a perfect light brown colour .I could leave it as it is and just ride it the improvement from 4000rpm would make it worthwhile but if I can tune this out I would be very happy regards baz

Carburetion is not tied to the RPM, it's throtle position related. So when you're getting these problems, are you full throttle on the main jet, or possibly rolling on (or off) the needle?? If you are at full throttle and getting stumbling, then do a plug chop and have a look.

Holmeslice said:

Carburetion is not tied to the RPM, it's throtle position related. So when you're getting these problems, are you full throttle on the main jet, or possibly rolling on (or off) the needle?? If you are at full throttle and getting stumbling, then do a plug chop and have a look.

Click to expand...

This is with the throttle wide open plugs look perfect cheers baz.

Have you tried tuning it using the Mikuni chart and a marked throttle?
I have done this many times, always has worked out perfectly.

http://www.sudco.com/Carburetor02.html

I believe 6500 rpm is a good safe red line for a Commando engine with stock bottom end and crank, particularly for 750s which do not have benefit of the strengthened cases and crank that came later, done for obvious reasons.

Certainly you will have reached Max HP unless many changes have been made to the engine. So why continue to make it rev higher? It is always gives greater acceleration to shift to a higher gear once Max HP has been reached.
I dont think the stock bottom end wants to run at 8,000 rpm for very many miles, but who knows the tipping point until it is too late?
So perhaps a flat spot way up there shouldn't matter.
Glen

bwolfie said:

Have you tried tuning it using the Mikuni chart and a marked throttle?
I have done this many times, always has worked out perfectly.

http://www.sudco.com/Carburetor02.html

Click to expand...

i have used this method before with twin amals and with my 34mm VM mikuni so yes its a good idea and it works i will give it a try thanks baz

concours said:

Flat spots are LEAN spots... you do the arithmatic :wink:

Click to expand...

In the UK we use the term flatspot to refer to a flatspot in an engines rev range this could be carburetion ignition etc i have never heard the term used exclusively to describe a lean condition in the carburetion so you live and learn cheers baz ps by "arithmatic" did you mean arithmetic ??

worntorn said:

I believe 6500 rpm is a good safe red line for a Commando engine with stock bottom end and crank, particularly for 750s which do not have benefit of the strengthened cases and crank that came later, done for obvious reasons.

Certainly you will have reached Max HP unless many changes have been made to the engine. So why continue to make it rev higher? It is always gives greater acceleration to shift to a higher gear once Max HP has been reached.
I dont think the stock bottom end wants to run at 8,000 rpm for very many miles, but who knows the tipping point until it is too late?
So perhaps a flat spot way up there shouldn't matter.
Glen

Click to expand...

yes i do agree with you 6500rpm is enough for a 42 year old engine i was just surprised when it started picking up at 7000rpm and just carried on revving but sympathy for the engine made me back off just before 8000rpm now looking at the engine there is a mist of oil around the crankcase joint at the back of the engine cheers baz

baz said:

In the UK we use the term flatspot to refer to a flatspot in an engines rev range this could be carburetion ignition etc i have never heard the term used exclusively to describe a lean condition in the carburetion so you live and learn cheers baz ps by "arithmatic" did you mean arithmetic ??

Click to expand...

Baz,

Is there any change at all with the different jets and needles?

You are correct that it might be something other than carb. I would rule nothing out, including ignition and plugs. It's hard to get a handle on what you are experiencing because "flat spot" can mean a lot of different things. Does it hesitate for a second? Does it bog down and then pick up? Are there any sounds that might indicate too much fuel or not enough? There can be a transitional issue between pilot circuit and main jet, but this doesn't sound like that because it is higher up in the RPM range.

The one "wrench" in the ointment here is the 40mm carb. It may be that the engine can't pull enough vacuum through the bigger carb and thus the response is slower...it has to catch up so to speak. This is a hard one because you have a special situation with the larger carb on there.

dennisgb said:

baz said:

In the UK we use the term flatspot to refer to a flatspot in an engines rev range this could be carburetion ignition etc i have never heard the term used exclusively to describe a lean condition in the carburetion so you live and learn cheers baz ps by "arithmatic" did you mean arithmetic ??

Click to expand...

Baz,

Is there any change at all with the different jets and needles?

You are correct that it might be something other than carb. I would rule nothing out, including ignition and plugs. It's hard to get a handle on what you are experiencing because "flat spot" can mean a lot of different things. Does it hesitate for a second? Does it bog down and then pick up? Are there any sounds that might indicate too much fuel or not enough? There can be a transitional issue between pilot circuit and main jet, but this doesn't sound like that because it is higher up in the RPM range.

The one "wrench" in the ointment here is the 40mm carb. It may be that the engine can't pull enough vacuum through the bigger carb and thus the response is slower...it has to catch up so to speak. This is a hard one because you have a special situation with the larger carb on there.

Click to expand...

the only real difference i noticed was when i had the needle in the richest position it was bogging down at small throttle openings as for the flatspot at 6500rpm it runs lumpy pulls back feels like its going to die then it hits about 7000rpm and starts to pickup its turbine smooth but i will blow it up if i keep doing it ,the plugs come out the right colour but i might try a richer jet needle just to see if its a transitional problem between the needle and main jet all though when i have done this the throttle is fully open so should be entirely on the main jet if it turns out the carb is simply too big then so be it cheers baz

baz said:

the only real difference i noticed was when i had the needle in the richest position it was bogging down at small throttle openings as for the flatspot at 6500rpm it runs lumpy pulls back feels like its going to die then it hits about 7000rpm and starts to pickup its turbine smooth but i will blow it up if i keep doing it ,the plugs come out the right colour but i might try a richer jet needle just to see if its a transitional problem between the needle and main jet all though when i have done this the throttle is fully open so should be entirely on the main jet if it turns out the carb is simply too big then so be it cheers baz

Click to expand...

Is it a hard "crack" on the throttle when this happens? That seems to be what you are saying. You said before you tried different "richer" needle. There are different main needle profiles that could get you a little more fuel but because this is at a certain spot in throttle likely won't make much difference. I'm really struggling because the typical symptoms don't seem to fit. If it was a fuel starvation problem (bowl running dry), it usually would take a bit before it caught back up. It's a little hard to follow that path with your description, but you might want to consider elimianting the possibility. You can set the float to max close position (point where it shuts off without hitting the casting). This would allow a little more fuel in the bowl. If the larger carb is pulling more fuel than what a normal 34mm carb would, you may need to look at the hoses and fittings to insure good fuel supply. Again those things are a bit far out there but worth a look...any restriction in flow might create a problem.

You didn't answer about the air jet. Mikuni carbs usually come without the air jet in the front of the carb. Do you have an air jet fitted to the carb? What size is it?

I think that if you have good pull through the range then the needle and needle Jet are good to go.
A smooth idle and coming off the bottom then the pilot jet is good to go.

When I started my main jet adjustment I was at 180. I went all the way to 195 then started punching them out with number drills. I'm sure I got up in the 230ish range. It ran fine with all these different but could not relieve the stumble during WOT at the 4500 to 5000 range. Rolling through was no problem.

I just could not get it through my head to go down but the only way to do it was to just get a range of main jets smaller then a 170. The 170 was the one I used to punch out with number drills because I just knew I would never have to go down. I was wrong.

Well I got an other 170, a 165, a 160, a 155 and a 150. Started running them down and felt improvement with the 165. EUREKA!
Put the 160 in and the world opened up and I knew the big gamble paid off. No more stumbles anywhere, freight train pull from bottom to top.

baz, keep going down with the main jet. You may have to go right to a 125 or 120 and you can split these by 2.5, as in a 122.5 or a 127.5. But then again, you may be just a size or 2 from that eureka moment. Remember, I was well into the 200's before I went all the way down to 160. And remember, my bike is not stock. Also going lower in the main will not harm that good pull you are experiencing at the other ranges.

It is possible and maybe likely that this carb is just too big, but I really think you are close.

Talk with your dealer and see if he will by back the ones you do not use.

dennisgb said:

baz said:

the only real difference i noticed was when i had the needle in the richest position it was bogging down at small throttle openings as for the flatspot at 6500rpm it runs lumpy pulls back feels like its going to die then it hits about 7000rpm and starts to pickup its turbine smooth but i will blow it up if i keep doing it ,the plugs come out the right colour but i might try a richer jet needle just to see if its a transitional problem between the needle and main jet all though when i have done this the throttle is fully open so should be entirely on the main jet if it turns out the carb is simply too big then so be it cheers baz

Click to expand...

Is it a hard "crack" on the throttle when this happens? That seems to be what you are saying. You said before you tried different "richer" needle. There are different main needle profiles that could get you a little more fuel but because this is at a certain spot in throttle likely won't make much difference. I'm really struggling because the typical symptoms don't seem to fit. If it was a fuel starvation problem (bowl running dry), it usually would take a bit before it caught back up. It's a little hard to follow that path with your description, but you might want to consider elimianting the possibility. You can set the float to max close position (point where it shuts off without hitting the casting). This would allow a little more fuel in the bowl. If the larger carb is pulling more fuel than what a normal 34mm carb would, you may need to look at the hoses and fittings to insure good fuel supply. Again those things are a bit far out there but worth a look...any restriction in flow might create a problem.

You didn't answer about the air jet. Mikuni carbs usually come without the air jet in the front of the carb. Do you have an air jet fitted to the carb? What size is it?

Click to expand...

this is with gently opening the throttle in second gear i had both fuel taps open i will check for any restriction i never said i changed the needle i changed the needle jet,i have not been asked about an air jet it does have one and i will remove it to see what size it is thankyou for your help regards baz

pete.v said:

Well I got an other 170, a 165, a 160, a 155 and a 150. Started running them down and felt improvement with the 165. EUREKA!
Put the 160 in and the world opened up and I knew the big gamble paid off. No more stumbles anywhere, freight train pull from bottom to top.

baz, keep going down with the main jet. You may have to go right to a 125 or 120 and you can split these by 2.5, as in a 122.5 or a 127.5. But then again, you may be just a size or 2 from that eureka moment. Remember, I was well into the 200's before I went all the way down to 160. And remember, my bike is not stock. Also going lower in the main will not harm that good pull you are experiencing at the other ranges.

Click to expand...

You might be right that he hasn't gone down far enough...but the plug burn says different...maybe

Your description of how you got there is the best way to do it. When I put my 34mm flat slide on I did exactly that. There was info on the web that got me in the ball park and I bought 3 mains either side of recommended in 1/2 sizes. I also have a hobby drill set I bought a number of years ago to size jets between the available sizes. I ended up on the smaller side of mean and had to drill it to get the full response...so it follows what you are saying.

Can't hurt to move down on the jet...just make sure you check the burn.

baz said:

dennisgb said:

baz said:

the only real difference i noticed was when i had the needle in the richest position it was bogging down at small throttle openings as for the flatspot at 6500rpm it runs lumpy pulls back feels like its going to die then it hits about 7000rpm and starts to pickup its turbine smooth but i will blow it up if i keep doing it ,the plugs come out the right colour but i might try a richer jet needle just to see if its a transitional problem between the needle and main jet all though when i have done this the throttle is fully open so should be entirely on the main jet if it turns out the carb is simply too big then so be it cheers baz

Click to expand...

Is it a hard "crack" on the throttle when this happens? That seems to be what you are saying. You said before you tried different "richer" needle. There are different main needle profiles that could get you a little more fuel but because this is at a certain spot in throttle likely won't make much difference. I'm really struggling because the typical symptoms don't seem to fit. If it was a fuel starvation problem (bowl running dry), it usually would take a bit before it caught back up. It's a little hard to follow that path with your description, but you might want to consider elimianting the possibility. You can set the float to max close position (point where it shuts off without hitting the casting). This would allow a little more fuel in the bowl. If the larger carb is pulling more fuel than what a normal 34mm carb would, you may need to look at the hoses and fittings to insure good fuel supply. Again those things are a bit far out there but worth a look...any restriction in flow might create a problem.

You didn't answer about the air jet. Mikuni carbs usually come without the air jet in the front of the carb. Do you have an air jet fitted to the carb? What size is it?

Click to expand...

this is with gently opening the throttle in second gear i had both fuel taps open i will check for any restriction i never said i changed the needle i changed the needle jet,i have not been asked about an air jet it does have one and i will remove it to see what size it is thankyou for your help regards baz

Click to expand...

Both the main air jet and the pilot air jet are blanked/plugged on these.