Combat detonation

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ILLF8ED

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In previous strings, I complained about the severe detonation problem I was having with my '72 750. Suspecting this was a carburetion issue, I switched to a single Mikuni. Amazing what a difference and confirms the Amals were running way to lean. No more pinging.
I plan to continue using the Mikuni while figuring out the problem on the Amals. I may be terminally old school, but still think the twin 932 Amals out perform a single carb. What I've done so far is strip and clean the Amals in an ultrasonic cleaner with lacquer thinner, then blew out all the jet areas with compressed air. The slides are in perfect condition having been sleeved by Mike Gaylord so mechanically there is no issue with worn parts. I'll assume the problem is setup and getting the correct jet sizes.
Been running 240 mains (230 is stock) #25 pilot, #3 slides (stock) and needle clip in the middle groove. That worked with the old gas....what jetting and needle position are you 750 combat owners using (Amals) with this gasahol that is getting good results.

Thanks,
 
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Normally detonation is caused by low-octane fuel and/or too much ignition advance. The twin Amals will make more power than the single Mikuni mostly due to the inefficiency of the 1-into-2 manifold as compared to two straight shots. Depending on the fuel available in your area, you may have to knock the timing back a bit to preclude knocking. Lean mixture can contribute to knock if the piston crown gets hot enough to ignite the fuel-air mix before the spark plug sparks.
 
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Detonation? You mean pre-detonation. While riding my machine, I get feedback of a marvelous controlled detonation. That is what this engine is all about. That being said, there are a lot of things that cause PRE-detonation. I think your carb set up is close enough. I have #26 and #27’s (73 combat carbs) and it is the same setup. Because your carbs are sleeved, I would look somewhere else.

1. Timing would be a good thing to check for pre-detonation.

2. Air leaks will cause the lean condition. The tough part is finding the air leak.

The carb manifolds are a very common source of unwanted air. While you have the carbs off, flatten all surfaces of mounting. I even pull the studs from the manifolds and run a mill file over them too. It is so easy to over tighten them so be careful. Get that oring to come home, then just a tweak tighter.

Another thing to check is the inlet valve seals on top of the guides. Sometimes people find that they are just not there. Bad valve guides are another source of air.

I am sure others will chime in with Lean/pre-detonation sources.
 

ILLF8ED

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pvisseriii said:
Detonation? You mean pre-detonation. While riding my machine, I get feedback of a marvelous controlled detonation. That is what this engine is all about. That being said, there are a lot of things that cause PRE-detonation. I think your carb set up is close enough. I have #26 and #27’s (73 combat carbs) and it is the same setup. Because your carbs are sleeved, I would look somewhere else.

1. Timing would be a good thing to check for pre-detonation.

2. Air leaks will cause the lean condition. The tough part is finding the air leak.

The carb manifolds are a very common source of unwanted air. While you have the carbs off, flatten all surfaces of mounting. I even pull the studs from the manifolds and run a mill file over them too. It is so easy to over tighten them so be careful. Get that oring to come home, then just a tweak tighter.

Another thing to check is the inlet valve seals on top of the guides. Sometimes people find that they are just not there. Bad valve guides are another source of air.

I am sure others will chime in with Lean/pre-detonation sources.

From Jerry's performance engine site:
Pinging, knocking and detonation are all terms for a condition known as "pre-ignition". That means some of the gas and air mixture is igniting in the cylinder before it's supposed to. And when that extra flame front crashes into the normal flame front (created by the spark plug), you hear a noise, and that noise is pinging.
"Detonaltion" is still a valid term.
From my previous strings, I mentioned I played with Ignition timing, and sorted out any ignition issues, leaving carburetion as the remaining vilolator. Your suggestion on air leaks is probably useful as it appears one of the carb intake o-rings was damaged.

What I'm asking for is a consensus on 932 contentric jetting other combat owners are using that is working. Thanks for your input on this. :)
 
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illf8ed said:
...what jetting and needle position are you 750 combat owners using (Amals) with this gasahol that is getting good results.

main jets: 230
needle jet: 106
needle position: middle groove

works excellent.
 
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illf8ed said:
[
From Jerry's performance engine site:
Pinging, knocking and detonation are all terms for a condition known as "pre-ignition". That means some of the gas and air mixture is igniting in the cylinder before it's supposed to. And when that extra flame front crashes into the normal flame front (created by the spark plug), you hear a noise, and that noise is pinging.
"Detonaltion" is still a valid term.
From my previous strings, I mentioned I played with Ignition timing, and sorted out any ignition issues, leaving carburetion as the remaining vilolator. Your suggestion on air leaks is probably useful as it appears one of the carb intake o-rings was damaged.

What I'm asking for is a consensus on 932 contentric jetting other combat owners are using that is working. Thanks for your input on this. :)
You are correct. Combustion is a controlled burn rather than an explosion(detonation). Pre-detonation is a common misused term.

Anyhow my 32 Amals are with a #3 slide, 220 mains, built in pilot(cleaned from the back side and blanked), needle in center groove.
I would think the 240 mains would be too rich and is probably compensating for another symptom.

Although my 750 with a combat head and BigBore exhaust runs well with this set up, I have enjoyeed a 36mm Mikuni single until resently when I experimented with a 34 mm Mikuni TM flatside. I may go ahead and cut the ambilical cord on the Amals for this is as close to 2 32s as I can imagine.

I do not know if this nullifies my posting but I run 110 racing gas 50/50 with 93 premium.
 
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Dosnt altitude need to fit in somewhere? I think Debby could help with that Phil

220 mains
needle middle
#3 slides
 

ILLF8ED

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Thanks everyone...stock setting it is. I'm in the SF bay area, so low altitude. Will go back to 230 mains and check for any and all air leaks.
 
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Im pretty new to these things so this might be right off the mark, being air cooled Im guessing the exhaust valves run pretty hot so is it possible that any carbon buildup on the valve could stay hot enough to make it detonate?
 

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Cheesy said:
Im pretty new to these things so this might be right off the mark, being air cooled Im guessing the exhaust valves run pretty hot so is it possible that any carbon buildup on the valve could stay hot enough to make it detonate?


There's actually a distinct difference between "detonation" and "pre-ignition"....

http://www.enginelogics.com/detonation.html

"Confusion and a lot of questions exist as to detonation and pre-ignition. Sometimes you hear mistaken terms like "pre-detonation". Detonation is one phenomenon that is abnormal combustion. Pre-ignition is another phenomenon that is abnormal combustion. The two, as we will talk about, are somewhat related but are two distinctly different phenomenon and can induce distinctly different failure modes"


"Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug."


"Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition."
 

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Continuing the string I started....the engine continued pinging with the Mikuni, just less. I went back to the Mike Gaylord sleeved original 932 Amals that I prefer. Resorted to 48oz of 110 octane leaded race gas per roadster tank full along with 91 octane unleaded pump gas. That cleared up the problem. Two weekends ago I pulled the head and did a de-coke and reassembled with the fresh composite type head gasket. Last Saturday I gave the engine a stress test after getting it nice and hot, put it in 4th gear at 30 mph and openned the throttle completely. No more pinging! After two more heat ups, retorquing the head, adjusting the tappets and resetting the carbs I nailed it in second gear up to redline. Now that's what I remember from my first combat owned in '73. At redline the power launches and it's scarey fast. Will enjoy it until the next issue comes up. :D
 
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Olk adding up all the feedback and what solved the 'ping', sounds like you had "pre-ignition" from the coke layer glowing hot setting off "burn" before its time.
Pre-ignition can lead to real "detonation" d/t over pressure of early "burning" with piston still approaching TDC.

Pump up water sprayer aimed into carb on hi idle or decent blip ups for some minutes can remove the carbon as greyish mess out the tail pipe. You can not hurt engine this way, most damage can be broken ankle trying to restart the fire from stalling it out until flushed out enough, just like over choking but less damaging as don't wash off bore oil and dilute oil. You can not hydro lock unless you put a full garden hose stream into the head.

Proper set up Combat is nice and normal spunky up to 6800 rpm, then the over size head ports and cam profile stuffs in mix over 100% efficiency to make mine feel like another piston kicks in. From 6800 on Combats can lift front straight ahead or leaned way over, whooWOO. Real shame as factory stuff just can't take that life style long. Built to take say 8000 rpm, nailing WOT after 7400 in 2nd can out leap 150+ sports bikes that wheelie too easy while long low Cdo just leaps forward, Ahhhhhh. Hey them 16,000 rpm sports are touching their red lines too to play at that level of get go. Just they got better alloys inside and geometry to carry on to higher top ends.

Also note that Nortons are fast burn hemi's so too much advance can tend to detonation as well as cut down on maxed out power. Might try testing time setting for best torque rather than highest advance it can tolerate.
Two Valve heads have innate swirl advantage over ordinary 4 valve unless 4 valve have staggered valve timing or sizes to create swirl rather than just mere tumble of charge, which stops tumbling as piston approaches TDC and mixture becomes as thick as honey.

hobot
 
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Oh yeah, my 2 Combat's liked the dual 32's better up to 5000-ish, then the bigger 34 single came out ahead. I prefer dual Amals on factory spec Combats up 120 mph but 34 single on special built light flywheel and 7 mm valves tall geared lighter weight 'Combat'. Slightly more need to twist up throttle and delay to get similar Amal response but then more pull for longer to at least 15 mph faster. ymmv.
Exhaust tuning has great effect on how big a carb a motor likes too, but as short as the split single manifold is, don't seem to really limit hi speed flows compared to dual 32's straight throughs. Single carb or single fuel injector/throat per jug needs to be a good bit bigger than a single inductor feeding multi cylinders via a common manifold. Assuming of course the manifold is built for speed and not just to fit ok.
 

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Hobot wrote:

I prefer dual Amals on factory spec Combats up 120 mph
---------------

Yes, my combat is per factory spec, but have never pushed it up to or beyond 120mph. Of course 120 is optimisitc if using a Smiths speedometer. :)
I typically don't rev higher than 6,000 rpms, so on the rare occasion getting it to redline, reminds me of being 20 years old and running my first combat. Jim Walraven rode my first Norton (combat)...was very impressed. Jim was the owner of the Jewel T class B altered drag car with 8 world 1/4 mile records.
 
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David,
I run stock setting in a pair of 932's on a 72 combat pictured to the right.
Top to bottom: 106 needle, clip in the middle position, .025 screw in pilot jet, 230 main jet.
Runs spot on.
93 Sunoco fuel, dash of 108 octane booster, 1/2 oz of lead additive, and 2 oz's of Marvels Mystery Oil (shaken not stirred)
Maybe snake oil but the queen digests it well :mrgreen:
Marshal
 
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My Combat did not detonate or ping on 91 oct but I did pull full all loads unless 93 oct in the tank. I don't ride my factory Combat Trixie to max as pure Norton stuff can't take it long and w/o the rods linkages its still plenty powerful to get into turns to hot to handle. Trixie gets limited as most everyone else, 6000 in lower gears just to get to traffic speed or bit above. 90 mph is about tops for me on Trixie though have run her up to over the ton some, just to see if she could. Trix came with 34 Micki carb but was factory so put on Peels new dual and was pleased that normal sane use was spunkier though an extra spring to work against.
Peel Combat had race level innards flywheel out, so not such worry to run up over 7000 on special occasions. Peel was powerful with the 28mm head on and allowed easy 120 cruise, with the 34 miki. Both carb brands were jetted as normal though I lifted needles on Peel to get a bit more umph out. Made Trixie too rich so dropped needles back to middle notch.
Combat are the cream of the crop to me, even plain factory versions please me a lot, even if not competitive to keep up with angry sports bikes like Peel. Who can live that style for long in public.
 
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For those of us who want to preserve the high-rpm pull of these engines, a dual-carb setup is the only way to go. I can't speak to the flat-side Mikuni, but the VM34 will only pass 6 oz/min of fuel max, and is thus limited to 35 HP max. Jim (Cosnoz) confirmed this a few months ago citing his real dyna data. The single Mikuni provides great low-end torque, but cannot support the horsepower that accompanies higher rpms.

Marshalnorton, you have a great set of buckhorn bars on your bike. As another fan of these bars, I have to ask, are these comfortable, or what???
 
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So which Cdo is expected to be faster at top end, smaller duals or a bigger single?
Say 2 x 32mm vs 1x 36 mm?

Seems pretty obvious that one 32 mm carb will not flow as much as two 32's, but I think that's because less stream lined Y manifold of a single carb.
Top end max power is traditionally gotten by shortest direct path of mixture into jug, so the chimneys of the big old mechanical fuel injectors.

Very pleasing for me to see the most maxed out top end racing Cdo's starting pretty easy and running quite well creeping in tight pits areas.
Did Norton or Amal short change us by not offering bigger dual or single carbs?
 
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Hobot,

The max flow rate I'm quoting for a VM32 is the raw fuel rate through the float valve when it is wide open. This rate sets the maximum horsepower obtainable with this carburettor, and assumes an efficient vaporization and vapor transport into the cylinder. Six oz per min corresponds to 35 HP max. My Victor uses an Amal 930 that flows 5 oz/min and is thus good for 30 HP, roughly where the bike was spec'd. The point of all this is that two 32mm Amals can flow 11-12 oz/min max, enough for 60 HP at the top end. A single VM34 will flow 8 oz/min or so, barely enough for 47 HP. It seems to me that, vapor flow issues notwithstanding, you simply can't get to higher HP numbers with a single carb.
 
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it is not that a single Mikuni cannot flow enough gas at higher rpm, it is that is cannot flow enough air.

Hobot, your bike will produce more horsepower and a higher top end using two carbs instead of one.
 
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