Need carb advise. Pumper carbs.!

pete.v · Apr 3, 2011

Need carb advise.
My 750 has had many upgrades over the winter. The main upgrade was a Stage 1 cam kit for JSmotorsport. Truly beautiful stuff. All things considered this is a great upgrade. Along with the crankcase swap out and all the machining the bike started on the first kick (Although all proper priming was done prior).

As with major changes like this, the main issue tends to be carburetion. It does run with the TM34 that was on it last year but very hesitant off the line and somewhat weak throughout.

I sprung for a set of 32mmPWK’s. Very strong yet idle was illusive and still with the hesitation off the line. I work very hard and long trying to get these going.

I set up a set Amal 33b and 34b’s perfect bores and chromes brass slides for the 750 replacing the split choke tube and got the 2 groove needles. After much jetting and needle fiddling I ended up change back the split choke tube and returned to the 4 ring needle with some success. Idle was attained yet still flat off the line. Mixing screw was only ¼ to ½ turn out. I decided to drill out the pressed in pilot and screw in a #30. A better idle was attained with both side even with 1 turn out. The bike start and runs well and at the top end it is out of site, Yet I still have somewhat of a hesitation when aggressive off the line. I have changed the pilot jet again to a 35 and will test again tomorrow.

With this cam setup and big bore pipe, I am thinking seriously about a 36mm to 40mm pumper to accommodate this aggressive setup. I know some have used them and richness was the main complaint.

What are some other experiences with pumper carbs (accelerator pumps)?

FYI, be on the lookout for a complete PWK flatside carb setup with extra jets and Mikuni TM34 that is phenomenal for a stock 750 in the for sale section soon. I am not going to burn my bridges till I get to where I am going (destination, fast and quick).

SteveMinning · Apr 3, 2011

Just my opinion, but I think going to pumpers to correct a mixture problem is the wrong approach.

Have you tried a different slide cutaway?

highdesert · Apr 3, 2011

No advice, but it is odd that you could not get the PKW flat slides to idle and pull right off the line.

I have a set on my 850, they start first kick, idle like a rock and pull hard with no hesitation all through the range.

I am sure you worked with Jim on this, various slow jet sizes, air screw settings and needle positions.

Feel free to PM or email me, I might have some ideas on setting them up.

BrianK · Apr 3, 2011

I run 35mm FCRs, which are pumpers. They will certainly cure any hesitation problems. Idle is not a pumper/non-pumper problem - that lies elsewhere.

I did find that the FCRs took a whole lot of dialing in when I got them, despite being "pre-jetted" for this application by an acknowledged expert. Had the same experience with a set of 41mm FCRs on my Ducati 900SS.

If you go the FCR route, PM me and I'll send you my ultimate jetting specs. They're for an 850 so I don't know if they'll work perfectly for you, but probably a good starting point in any event.

Best of luck whatever you do.

pete.v · Apr 3, 2011

SteveMinning said:
Just my opinion, but I think going to pumpers to correct a mixture problem is the wrong approach.

Have you tried a different slide cutaway?

Having #3's slides now, which direction would you suggest? Right now the plug are a nice light straw color and the idle is quite fair for hotter cam. My only issue now is that hesitation during heavy quick throttle.

pete.v · Apr 3, 2011

highdesert said:
No advice, but it is odd that you could not get the PKW flat slides to idle and pull right off the line.

I have a set on my 850, they start first kick, idle like a rock and pull hard with no hesitation all through the range.

I am sure you worked with Jim on this, various slow jet sizes, air screw settings and needle positions.

Feel free to PM or email me, I might have some ideas on setting them up.

Ya, I know we spoke a week or two ago. I am a little disgruntled with the manufacturing of the PWK's. Quite inconsistent from carb to carb. This serves only to magnify an already delicate setup. The Amals were a breeze in comparison and the performance is not that far off either. Not what I expected. I do not blame the dealer of these for I know that no 2 bikes breath exactly alike.

AussieCombat · Apr 3, 2011

G' day pvisseriii... My experience is that we don't always need big carbs to make things go better, sometimes they tend to make things worse than they actually are. I had a very fast CCM 500cc back in 75 that I ran in motocross, hillclimb and dirt track including mile speedway that ran a Mk 2 Amal @ 34mm, and a bored and stroked TT 500 Yammy that would pull higher gearing with a 36mm VM than a 38mm VM... However,... If you have got it idling o.k. your next step is Slide Cutaway and Needle position. A "flat spot" generally means there is a leanness of fuel. Try the needle up one, that is, raise the needle by lowering the clip. If it becomes too rich then you may need another needle with a finer taper, or needle jet, or both. If this helps, all well and good, if not, return the needle, and then try a slide with less cutaway, that is, allowing less air...you might be expecting too much in the way of snapping the throttle open, big carbs will do that. but you'll get there. It's all good fun. Hope this helps..AC.

comnoz · Apr 3, 2011

I would try installing a balance tube. That will change the crossover point from the idle to the needle circuit and may be just what you need. It is often overlooked as a tuning aid. I have installed a small valve in the crossover tube to fine tune the crossover point. Or just give it a try with about a 1/8 ID fitting. Jim

xbacksideslider · Apr 3, 2011

Ditto on the balance tube idea. I have a pair of PWKs from JS Motorsports too. They're great. I did put them in the drill press and drilled them for balance tubes that I fabricated from some steel brake line; barely flared the outside/top end to ensure a good seal to the balance pipe tubing and to ensure that it can't fall into the engine; also, just a dab of JB Weld to ensure that the tubes' press fit stays vacuum tight.

I'm sure you've tried it already, but it bears saying - you might try richer pilot jets.

Ditto on the pumper idea too. Dellorto pumpers fixed cammy low speed hesitation on Ducati singles for me. They're too big bodied for the Norton's close quartered intakes so the FCRs are the next step, just too darn pricy.

Carbonfibre · Apr 4, 2011

Firstly you need to find out if the carbs you have are OKO's or genuine Keihins (the pic on the JS website looks like OKO's to me!). Next remove the carbs and carefully check the float levels, and although your problem doesnt seem to be related to carbs, increasing the size of the OKO air corrector drilling to 1mm will improve low speed throttle response. Needle position is also crucially important on these carbs, and glitches and hesitation can often be eliminated simply by changing the needle clip position.

comnoz · Apr 4, 2011

BrianK said:
I run 35mm FCRs, which are pumpers. They will certainly cure any hesitation problems. Idle is not a pumper/non-pumper problem - that lies elsewhere.

I did find that the FCRs took a whole lot of dialing in when I got them, despite being "pre-jetted" for this application by an acknowledged expert. Had the same experience with a set of 41mm FCRs on my Ducati 900SS.

If you go the FCR route, PM me and I'll send you my ultimate jetting specs. They're for an 850 so I don't know if they'll work perfectly for you, but probably a good starting point in any event.

I don't understand how anyone could sell a carb for a motorcycle and say it was pre-jetted. It must just mean the jets are installed because I haven't seen two Nortons in a row that worked best with the same jetting. Minor tolerances in manufacturing both in the engine and the carb are likely to need jet changes as well as altitude changes and the many modifications that have been done over the past 30 years.
Might get away with it if it was a bone stock late model Jap bike but a Norton- never. Jim

pete.v · Apr 4, 2011

comnoz said:
I don't understand how anyone could sell a carb for a motorcycle and say it was pre-jetted. It must just mean the jets are installed because I haven't seen two Nortons in a row that worked best with the same jetting. Minor tolerances in manufacturing both in the engine and the carb are likely to need jet changes as well as altitude changes and the many modifications that have been done over the past 30 years.
Might get away with it if it was a bone stock late model Jap bike but a Norton- never. Jim

Man! I here that. They should come with a loaner jetting kit and only pay for a deposit and for what you use.

Thank god for those little quick disconnects for the fuel lines. Saves the petcocks from getting munged up from the pliers.

OK, I am going with the balance tube between the carb and give the PWK's another go around.

Clip change difficulty, Amal fairly easy, PWK not too bad, Mikuni TM rather a pain (although I have become quite proficient at all three).

pete.v · Apr 4, 2011

Carbonfibre said:
although your problem doesnt seem to be related to carbs, increasing the size of the OKO air corrector drilling to 1mm will improve low speed throttle response. Needle position is also crucially important on these carbs, and glitches and hesitation can often be eliminated simply by changing the needle clip position.

I have run the gammut of clip position and have made changes to the pilot jetting. If not carb, what do you think my problem may be? Timing is perfect no valve clash and valve timing on although I do have greater lift and duration.
And just what is the air corrector you speak of drilling to 1mm?

Carbonfibre · Apr 4, 2011

Firstly did the bike run ok before the changes were made? If so how much difference between the stock cam and the replacement? How was the cam timing checked, and if there was any need for adjustment how was this carried out?

pete.v · Apr 5, 2011

Carbonfibre said:
air corrector drilling to 1mm will improve low speed throttle response.

What is the "air Corrector" that you speak of.

pete.v · Apr 5, 2011

Carbonfibre said:
Firstly did the bike run ok before the changes were made? If so how much difference between the stock cam and the replacement? How was the cam timing checked, and if there was any need for adjustment how was this carried out?

No adjustment was needed only validated with a degree wheel.

pete.v · Apr 5, 2011

Carbonfibre said:
Firstly did the bike run ok before the changes were made? If so how much difference between the stock cam and the replacement? How was the cam timing checked, and if there was any need for adjustment how was this carried out?

bpatton · Apr 5, 2011

pvisseriii said:
. Very strong yet idle was illusive and still with the hesitation off the line.

pvisseriii said:
My only issue now is that hesitation during heavy quick throttle.

Can you narrow it down to a specific throttle opening range where you get the hesitation? Do you mean 'tip in' or WFO?
The hesitation off the line would be like idle to 1/4 throttle, 1/2 throttle?
Heavy quick throttle starting from anywhere to WFO?
In other words, what's the throttle opening when it first shows up? Does it actually stutter or is it just flat?

Hobot · Apr 5, 2011

I'm thinking the slide opening may be next thing to diddle as very common in hot rods with bigger carb to drill air bleed hole in butterfly plate to get enough air motion to suck up fuel so they don't have to spin to 2000 to avoid stalls.

Here a V8 site that covers the concepts and basics to be aware of. Basically they try various ways to get air flow enough to suck up fuel enough and diddle timing to get high rpm smooth idle then back off the throttle hold open screws till idle as desired.
http://www.clubcobra.com/forums/showthread.php?t=71440

Carbonfibre · Apr 5, 2011

Cant really advise without any information on the numbers related to the new cam, or how the timing was checked. But in some cases where intake depression at idle is reduced by a longer duration cam, if you have OKO carbs then drilling out the air correcters (small holes in carb intake throat, that will be revealed when pilot screws are removed) will often correct poor idling.

Wonder where you bought the carbs from though, as I would have thought that the seller should be able to help with this, as if the problem is linked to the carbs then its probably something that has happened before? I have noticed a few people selling the OKO carbs have been removing the OKO branding from the carb bodies, and selling as Keihins though, and not sure whether you are likely to get much help from a seller like this.

Need carb advise. Pumper carbs.!

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