Carb Blowback?

[amatco]

Hi everyone,

I have a 81H Venture. I recently ground the valves and installed new rings. I now have good compression. I cleaned the carbs and re-assembled the engine. The bike starts good and runs fairly good however the carbs intermittently blowback. Sometimes a vac hose will blow off. I sycnt the carbs with mercury guage, I do not know if I should clean the carbs again, again and again. I did replace the airbox with individual air filters on the carbs. It is now easier to remove and install the carbs. On the 81H model you cannot set the timing. The Idle/air mixture jets are fixed and it doesnt seem to make any difference how the adjusting needles are set. I want to fine tune this engine but I am not sure what to do next. The pickup coil wires seem to be OK. The coils are at 3 and 15K ohms (seems like a weak spark). The Iginiton box "who knows?" Everything has to work together and something is just not right!
If anyone has any ideas I would appreciate it. Thanks, Allen

[Ken Talbot]

Sounds like you may have inadvertently put it back together with a cam one tooth out of sync. You could check this by removing the cam cover and the timing cover, using a dial gauge in #1 to verify TDC, then checking the cam alignment marks.

[strom]

I agree with Ken on this one.

LP

[amatco]

Thanks guys,

I did not think I made a mistake with the cam chain but I removed the covers and double checked the timing marks at TDC. My blowback problem is not all that bad but I know it can be solved. I suppose a worn cam chain and wear on the cam itself can contribute to the problem. Short of degreeing the cams by slotting the sprockets I am still not sure what to try next.
So, I am open to any imput anyone might have. Thanks again, Allen

[skids]

Re: Carb Blowback?

I had a set of carbs that were doing that when the baffles were shot (read LEAN).

I also had a set of 78 carbs that were intermittantly backfiring and which seemed like they would not richen with the pilot screw adjustments. I installed 4-1 Jardine pipes on that bike with K&N washable filter).

The trick is to make sure you have big enough main jets (and of course float heights adjusted). I have experimented with different pilot jet sizes for different exhaust configurations, but have always gone back to the 42.5 genuine Mics (see Dennis Kirk catalog for new). I should at least tell you that I have not tried individual filters yet. Lean idling WILL backfire like that and bigger jets can certainly help. Try at least 2 main jet sizes bigger with each try. I'll bet your bike won't even do 100 mph in the condition it is in.

There is a good article about carb adjustments for our CV carbs. Start with the main jet sizes. They suggest midrange next, but with the 81H carbs, you should (my opinion) then go to the idle circuit because there are no slots to adjust the main jet needle height. Adjust pilot screws with a colortune next if you can get your hands on one. If your carbs idle screws are too far from 1.5 turns out, and still not getting rich enough, you might try bigger pilots, but you should give some thought to even bigger mains.

Originally posted by amatco
Hi everyone,
I have a 81H Venture. (snip)The bike starts good and runs fairly good however the carbs intermittently blowback. Sometimes a vac hose will blow off. I sycnt the carbs with mercury guage, I do not know if I should clean the carbs again, again and again. I did replace the airbox with individual air filters on the carbs. It is now easier to remove and install the carbs. On the 81H model you cannot set the timing. The Idle/air mixture jets are fixed and it doesnt seem to make any difference how the adjusting needles are set. (snip)
If anyone has any ideas I would appreciate it. Thanks, Allen

[skids]

Re: Carb Blowback?

PS, you probably have a Venturer, not a Venture. =;-)

Originally posted by amatco
Hi everyone, I have a 81H Venture.

[amatco]

Skids, Thanks for the reply.

You mentioned -I had a set of carbs that were doing that when the baffles were shot (read LEAN).

You refer to baffles - Do you mean the diaphragm assemblies? When I cleaned the carbs I noticed that the diaphragm (rubber) had some pinholes and wear spots. I repaired this with a special rubber compound. I think it was successful.

I am going to follow up on some of your suggestions. Thanks for the info.

Allen (amatco)

[amatco]

Skids,

Me again.

I guess you were talking about the mufflers. With the baffles out it would mean less back pressure. This makes sense. I will check this out also. If I need new mufflers will any add on with the same outlet be OK. Or do I have to be carefull ? Please advise if you get a chance. Thanks, Allen (amatco)

[skids]

Sealing the holes in the carb diaphragms will help! I was referring to "baffles" in the mufflers. Exhaust modifications can be a trial and error affair. Harley muffs can be used on your pipes but you may experience some loss of pulling torque at about 3000 rpms, and then they run better at the higher rpms.

I had a set of Emco (?) muffs from JC Whipme that caused me so much grief, that I gave up on them. I couldn't believe that different muffs could make such a difference.

Manufacturers go through a lot of engineering to get just the right performance out of exhaust systems. The first Honda VTX (1800 cc) bikes had mufflers that looked like chrome-plated trash cans (ugly), and they did that strictly for added performance, I am sure.

Originally posted by amatco
Skids,

Me again.

I guess you were talking about the mufflers. With the baffles out it would mean less back pressure. This makes sense. I will check this out also. If I need new mufflers will any add on with the same outlet be OK. Or do I have to be carefull ? Please advise if you get a chance. Thanks, Allen (amatco)

[schpiff]

I am no expert..but back fire thru the carbs usually means a lean condition......or retarded spark..or exhaust not opening. Most of the xtreemes know more than I on these engines tho...
I am not sure if its at all possible to even get it to running condition with the intake cam 1 tooth off. Again I would trust them!

[Jeff]

The current mufflers on my 78 XS1100E are completely gutted ie. no baffles what so ever and I experience both exhaust and carb backfiring which has on occasion popped off the vacuum line to the right petcock. Due to the lean conditions caused by these existing mufflers and still running all stock jets.... I had to run my mixture screws at about 3 and a half to 4 turns instead of the customary 1 and a quarter to 2 turns.
I should be getting my new Jardine slip-ons later this week to fix this and definately quiet the bike down a bit. I do admit that it sounds mean with the wide open exhaust but it is loud to the point of being obnoxious. I will report back with what I find after replacing them with the new Jardines.

[sixtysix]

I just had this problem on a Suz gs850, found the cyliders were not firing all the time and this would let the raw gas in to exhaust for a nice pop back through carb. I figured it was carbs!