Author Topic: Can too much back-pressure in an exhaust system cause the engine to die  (Read 2010 times)

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Offline Steveola

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I have been having trouble with my 1975 CB750 (actually 836) dying after it gets hot and refusing to turn over. I have been thinking it was the carbs but I am thinking that it may be my exhaust. The 4-1 system I have been using if a Jardine from the 70s. I got it from ebay and it looked like it was too small a diameter pipe but it fit,so I put it on. It always starts fine and runs good until it gets hot and then acts like it has vapor locked. It just occurred to me that it could be the wrong size header causing too much back pressure. Any help is greatly appreciated. By the way, I just ordered an exhaust from Motogpwerks.



Offline rboe

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Interesting theory but it would be a first for me. Depending on how you routed the fuel line the heat may be boiling it in the line and vapor locking there.

I'm not all that experienced with hot dying engines but my choice in your case would be faulty coils. The heat can cause the coils to fail once the heat expands it breaks the circuit; although it typically is not sudden. But there you go, a cheap Internet opinion. :)

Too small of an exhaust WILL choke off your engine so it can't breath that well. I gained 12hp going from a too small Super Trapp back to the stock can on my Quota (dyno'd it with both pipes so I have the data to prove it)
1974 CB350F
2008 XR650L
2012 Griso SE

Past iron
1971 Suzuki TS125
1977 Honda CJ360T
1981 Kawasaki 550Ltd
2000 Moto Guzzi Quota



Offline cougar

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Steveola ;  Yes! With a completely (properly) sealed exhaust, you can block the outlet(s) and kill an engine.  ...cougar...
I'm not prejudice, I'll weld anything that pays! Knowledge that is shared is Never Lost!!   Right is right, wrong is wrong! The truth is the truth and a lie is a lie! DEAL WITH IT ACCORDINGLY !!!   I HATE "DIAL-UP"



Offline Don R

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My exhaust was blocked by the PO and his roll of pink fiberglass. He wrapped over the inlet end of the baffle inside. the engine was loaded with carbon. It would only run a few seconds. A jardine will not cause this problem unless it is bashed in on the bottom or plugged. They were one of the better ones when new.
The wheel keeps on turning and you cant slow down, you can't hold on and you can't let go, you can't go back and you can't stand still, if the thunder don't get ya then the lightnin will.



Offline ekpent

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Expound a little on the 'vapor lock' when hot.Does the engine not want to turn over with both the starter and the kicker like maybe its showing signs of a mild seizure. A combo of the timing being off and the jetting being too lean on the 836 kit could cause some big problems like overheating.  When running you should be able to feel behind the exhuast outlet with your hand to see or feel if the pipe is blocked. A peek at your spark plug color will also hold clues.



Offline Steveola

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I can feel exhaust coming out of the end of the megaphone. It has the stock baffle. The timing seems to be right on. I have gone to 110 and 40 jets. I checked the plugs and they are brown/tan in color without fouling. I am running pods but it never needs choke to start cold. I installed a Dyna S ignition with new 3 ohm coils it calls for in the kit. I sooooo confused. Once it has gotten warmed up it will die at idle and won't turn over. I can't use the kick start due to rear sets, but the starter won't turn it over. It almost acts like the battery is dead but it's a new OEM BATTERY. It will start right up after it has cooled down. I pulled the carbs and checked the float level adjustment. All okay. HELP!!!!!!!



Offline ekpent

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Sometimes with an 836 kit with higher compression a weak starter can struggle. That combined with the engine hot can create even more drag on it.  If you have that kickstart still see though if you can mount it just enough to see if the engine still turns over fairly easily when it happens again,doesn't need a full stroke to tell.
  But you say it dies at idle when hot-Hmm. Dyna'a have been known recently to be problematic in some cases,had a bad run or something,have read complaints.
  Sounds like your plug color is good but I run a larger main jet on my 836. Your #110 with a kit,pods and a pipe sounds a little puny. With just pods and a pipe it is sometimes best to up the size.
  My advice is to make sure your not overheating and coming close to a seizure. If the heat is affecting your Dyna,causing it to die, which is possible it still should not affect the starter loop.If you still have the points set up around you could pop those back in to compare,
  How long has it been on the road so far ?  Have you ever heard any pinging etc. while riding under load ?
« Last Edit: June 05, 2012, 08:10:04 AM by ekpent »



Offline Rgconner

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3ohm without resistors?

The problem may be overheating wires. Start checking the loom for melting wires.

They get hot, resistance goes up, power goes down and the engine stops.

Also evidenced by not enough juice to turn the engine over. Check that voltage!

Sitting at idle will drain the battery, especially with a Dyna coil and 3ohm coils.

1975 CB550K aka "Grease Monkey"



Offline mycb750k6

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I had that sticking problem with both my 836 (otherwise known as cheap ebay $100 836 kit) and my cycleX 850. Problems go away in time but the cheap 836 was more problematic. Not to hijack the thread but I was going to offer a comparison between the two sometime and I'll say now there is NO comparison. The cycleX coated 850 domed pistons are amazing and provide significantly more power/torque than the 836. Anyway, that's been my experience. I'm considering replacing the 836 with CycleX stuff.



Offline Steveola

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The 836 kit was from Wiseco. So i think it' s okay. I haven't noticed any pinging at all. Actually it runs pretty good until it gets hot and dies at idle. I have the gas tank off right now so I can inspect the wiring pretty easily. I have a set of OEM coils I don't have the points though. Doesn't Hondaman have an ignition setup? Does this sound electrical and not carbs or exhaust? Thanks everybody I really appreciate all the help.



Offline ekpent

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"Actually runs good till it dies" is not an answer to get much help. How long have you been riding it. Is this a problem that has just developed after a long tenure or is it something that has been happening since you just recently built it and have first put it on the road. Guess I will have to go back and read again or just say good luck.



Offline Steveola

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The engine has about 55 miles on it since the top end rebuild which consisted of an 836 big bore kit, Manley valves - std diameter, webcam, valve springs, alloy keepers, HD cam chain and the roller assembly. I also replaced all the seals when I  had it apart. This has been happening since I started riding it. I have been working out a few bugs and trying to get the carbs dialed in, things like that. 



Offline Stev-o

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"things like that"?   What does that mean?

Have you given it the full 3000 mie tune up including valve adj, timing, and carb sync?
'74 Honda 750K [836]   '77 Honda 750K project    76 Honda 550K cafe      plus plus plus



Offline Don R

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How much piston to cylinder clearance is there?
The wheel keeps on turning and you cant slow down, you can't hold on and you can't let go, you can't go back and you can't stand still, if the thunder don't get ya then the lightnin will.



Offline ekpent

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The engine has about 55 miles on it since the top end rebuild which consisted of an 836 big bore kit, Manley valves - std diameter, webcam, valve springs, alloy keepers, HD cam chain and the roller assembly. I also replaced all the seals when I  had it apart. This has been happening since I started riding it. I have been working out a few bugs and trying to get the carbs dialed in, things like that. 
With all those nice go fast goodies you have in there those #110 main jets still make me a little nervous. The early 70-71 750's came with #120's stock from the factory back then. Air cooled engines rely heavily on gas to help them run cool,not just air.



Online PeWe

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Try larger main jets. 110 must be toooooo small :o

My bike had 105 from factory (K6 with 63hp).
If I remember correctly, these modifications were made 1980-83.
4-1 and OEM air filter box 115. 
4-1 with KN separate filters 120-125.

I'm sure about this
836cc + ActionFours SS-1 (mild cam), ported head with larger inlet valves (34mm), . Mainjet 145 + throttle needles one step down from OEM setting (needle will lift and give more fuel at 1/4-3/4 throttle).

My first trip on Autobahn was made with a 4-1 with a too restrictive baffle and last engine setup. The 4-1 was delivered with 2 baffles, loud and nice + very silent. I did not test the silent one before the journey. On Autobahn when I needed to go faster than 120km/h I got problems when the max speed was 130km/h and the engine sounded as it should explode in any minute. Engine did not stop at idle and very warm for that.

I had to drive with no baffel at all... nice song in 160-210km/h. Mostly around 180km/h. Lots of people heard us!! ::)
...all the way to former Jugaslavia, next over the sea from Venice. Then back home to Sweden again (total 4000km, 3000 without baffle). I had a nice ringing sound in my ears over a week after. My helmet then was not so insulated as later helmets. Jebs Clay Regazoni was nice helmet then but not so much damping soft stuff as later helmets.
« Last Edit: June 06, 2012, 09:30:33 AM by PeWe »
CB750 K6-76 with modified engine.



Offline crazypj

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The engine has about 55 miles on it since the top end rebuild which consisted of an 836 big bore kit, Manley valves - std diameter, webcam, valve springs, alloy keepers, HD cam chain and the roller assembly. I also replaced all the seals when I  had it apart. This has been happening since I started riding it. I have been working out a few bugs and trying to get the carbs dialed in, things like that. 

How much piston to cylinder clearance is there?

I think Don R has it.
Wiseco's need a lot more than stock clearance
Your jetting also sounds way lean, are you running stock airbox?
I fake being smart pretty good
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Offline Steveola

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Not sure what piston clearance is. My machinist who builds quite a few race engines did the work on the cylinders/pistons and gapped everything. I have alot of faith in him. He is fairly well know in the vintage racing world. I have 40 main jets, and had 115 pilots in at first. It seemed a little rich. I agree that 110s seem small. I changed that on the advice of my mechanic. I rode it once about 10 miles to check it out and it seemed to be a little lean although it didn't really stall or stumble during acceleration. As a matter of fact , while I was riding it I engaged the choke and it bogged down quite a bit. When I turned off the choke it cleared up. I haven't pulled the plugs but I had to take the exhaust header off to paint it and the ports looked slightly black with a tan color on the exhaust valve stem. I not really comfortable running that small a jet so I may put the 120s back in or go up a size beyond that and ride it for a bit and check the plugs after every few runs. One thing that made me think it might be rich is that you never have to choke it when cold, but according to the manual for this carb number the mains are correct. I checked the float level adjustment and it seems okay. By the way I am running pod air filters from Cycle X. I am starting to think it might be a collection of things. Things that by themselves wouldn't really impact performance but together cause problems. In the meantime I have sprung an oil leak  ( the rubber nickel between 2 and 3 cylinder has started leakingI think) that will require the engine to be pulled out so I need to check the carbs too.



Offline crazypj

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That jetting is way lean with pods.
I'll bet you raised the needles as well? (4~5 clip from top?)  ::)
I fake being smart pretty good
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Offline Steveola

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Where should the needles be set. What is the OEM setting. Is that in the manual somewhere? Based on what everybody has said and tonight I closely inspected the exhaust ports and it is waaaay lean, The no. 4 exhaust valve stem is almost white. Where could I find a selection of jets?

 

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