New exhausts poor running

PM999S said:

I recently installed new exhausts on my '72 Combat using baffleless peashooters.

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PM999S said:

Initially, I jetted up to 280 from the 230s that were standard to compensate for the reduced back pressure and went for a quick test ride.

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L.A.B. said:

Which peashooters (or other silencers/mufflers) were fitted beforehand as I would have expected them to also be the same baffleless type?



The main jets for a 750 Combat with standard (06-1978) peashooters would be 230 (260 for 850).

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ashman said:

I built my 850 to Combat specs, 2S cam, ports and shaved head work as well a open exhaust system, I run 270 main jets in my Amals, needles set on the bottom notch and with the Joe Hunt maggie my Hot motor runs great and has been set up this way for over 40 years now, I replace the needles and jets every 3 years as they do wear out from vibrations, make sure the manifolds aren't leaking or any air leaks and float levels are right, I have also ran with a K&N filter for the 40 years.
Because your plugs are running clean mean nothing really as its hard to judge plugs with todays fuels, maybe you have over jetted it, 300 main jets might be a bit high.
But of course mine is a hot 850 and not 750 as the Combats are and 300 jets might be just to much.

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PM999S said:

While there are loads of threads on pilot/mixture settings, my problem doesn't seem to fit with the many very good suggestions. I recently installed new exhausts on my '72 Combat using baffleless peashooters. Initially, I jetted up to 280 from the 230s that were standard to compensate for the reduced back pressure and went for a quick test ride. The bike certainly revved up quicker although I was still unsure whether the jetting big enough albeit the plugs have never been cleaner. Partial closing of the throttle didn't markedly affect the acceleration so I thought things might still be a bit lean.
This is where things started to go wrong, because I decided to try out the 300 jets and change the aging filter with an unused K&N for the standard airbox from the 1970s that I found on eBay to see if it improved breathing even further. However, while the acceleration remained strong, closing the throttle set off coughing and spluttering to the point of popping and backfiring. So, I went back to the original settings complete with the old filter but the problem remained.
I then looked for air leaks and saw the previous owner had tried to repair the carb rubber hoses with a bike tyre repair kit. Certain that this was the issue, I fitted new hoses after carb-clean spraying the pilot/mixture channels and checking the seating of the floats. No discernable difference. Speeding up the idle and richening the mixture screws have helped a bit but not massively. I've also checked the seating of the new headers, secured by new brass roses and gaskets, and there is no movement there to let air in. What have I missed?

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marshg246 said:

I regularly install Amal PACK 140 on Combats with no changes from the Amal setup except I use #19 pilot jets. Most likely the same peashooters as you have (LOUD), hamcan, close to sea level.

Model: Commando Combat
Right Carb: R932/19
Left Carb: L932/20
Main Jet: 220
Needle Jet: 106
Throttle Valve: 3
Pilot Jet: #19
Needle Position: 1
Needle: Std
Spray Bar: Std

So far, I've found no reason to change. I have tried the needle in the middle mostly to keep the engine a little cooler but haven't noticed any significant change.

So, I think you have WAY to big a main jet. If the back firing is a really loud bang, it is not actually back firing but rather explosions in the exhaust pipes cause by being over rich.

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PM999S said:

Thanks for the reaction but the issue didn't manifest itself when the straight-throughs and 280 jets were first fitted - the bike breathed well enough on both open and closed throttle. It was only after I continued to muck about afterwards that things went wrong and I can't get it back to that first test ride...

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marshg246 said:

Sometimes you just have to not worry about logic - too many variables. Make it stock, make sure the slides and needles are moving up and down properly, and give it a try.

There is no doubt, as far as I am concerned, that you are way rich on the main jet. But I can't tell from your writeup if that matters. The main jet is only really important from about 3/4" throttle to WOT. You also said that the air screw helped - that has no affect at more than about 1/8 throttle.

So, tell us the symptoms at idle, 1/4, 1/2, 3/4, and full throttle

If you've never seen/read this: https://amalcarb.co.uk/downloadfiles/amal/amal_tuning_guide.pdf

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JimNH said:

Nothing to do with this problem other than terminology so that it's easy to follow. Backfiring is back through the intake while after firing is out the exhaust. It sounds like this may be fuel in the exhaust.

I've always thought of it as backfiring = air, looking for fuel; after firing = fuel, looking for air.

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PM999S said:

It's definitely the latter, with popping through the exhausts not the air intake as far as I can tell on the run, and only on closed/feathered throttle as everything is smooth and strong on the open throttle

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marshg246 said:

So, dumping enough fuel in with a closed or slightly open throttle to get exhaust bangs! Not sure how we started on main jets! Sounds like a stuck/miss-adjusted float(s) or chokes on. Do you have chokes?

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Tornado said:

Could carb sync or throttle balance have been compromised during your messing about? Check the tiny o-rings on the idle air and throttle stop screws. If there is a damaged one it will leak air and mess up idle control. What about choke slides if present? Could they be in closed position by accidentally putting control lever wrong direction or just stuck down?

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