Not Running: Backfire/Puffing gas through carbs on start up

[Rogue21]

I am trying to get my project running for the first time and have been having the issue of what seems to be a back flow of gas through the carbs and out the air filters. I am not leaking gas, but I will get the occasional puff of fuel out the air filters when starting and do not know if this if normal. After attempting to start, the inside of the carb bodies (venturi) is coated with fuel.

The bike is also firing on the left cylinder onlly (I switched the left and right spark plug wires and no change occurred). I am wondering if there is a connection between gas in the carb bodies and no ignition in the right cylinder?

Engine is a 1978 with a set of '78 BS38 carbs. Here is the list of what has been done so far:

Engine Rebuild - Bored cylinders, new pistons, new rings, new cam chain, new cam chain slide/tensioner, valves grind and lapped.

Carbs: Cleaned twice, new gaskets, new float valves and seats, Floats set to 24mm, #45 Pilot Jets, #142.5 Main Jets. Synchronized but not balanced, mixture screws are 3.5 turns out.

Cam chain tension was set, valves then set to 0.004" on intake and 0.006" on exhaust.

Currently running pod filters and just headers for the exhaust pipes.
Running Hugh's PMA, with a Pamco Ignition Unit, and a Sparx capacitor.
I've attached a picture of the current wiring set up. Wiring is 14 gauge, connections are either soldered or securely crimped with good ground connections.

BTW, its about 30 degrees F here..Thanks in advance for the help!

 

[5twins]

The pilot jets and mix screw settings don't sound right for a '78 carb set (are you sure that's what carbs they are?). The usual pilots are a #30 or #32.5 VM22/210 type with mix screw settings around 2.5 turns out. A 45 pilot is huge for that carb set and could be partly, if not wholly responsible for your symptoms.

 

[Rogue21]

5Twins - I am almost certain the carbs are '78s, but you are correct about the jets/mixture screws. I installed a #45 because of the air filters and non-baffled exhaust and thought I needed more fuel flow because of the extra air flow. I have two more sets of pilot jets somewhere in the 30s range that I will install and tryout.

 

[5twins]

Yes, you do usually need more fuel for those mods, but not that much, lol. The stock pilot for your carb set was a 27.5. The 30 and 32.5 are one and two sizes up. That's all that's usually required. Also, make sure you are using the correct pilots. Your carb set was designed to use the VM22/210 type. The other type, the BS30/96, looks the same externally and will physically fit, but they flow differently and won't work right in your carb set. Here's how to tell the two types apart .....

 

[Rogue21]

I just installed a set of VM22/210 #30 pilot jets today. After many kicks and many adjustments, I now have the left cylinder running strong at idle (still could use some tuning and adjusting), but the right cylinder still has nothing!

I tried adjusting the right carburetor mixture screw with the bike running, but didn't get a single pop from the exhaust. I got a few backfires through the carbs and the right air filter was soaked with fuel. The left air filter had a little fuel, but not much.

My next plan is to run a compression test and see what numbers I get back. I really dont want to dig into the heads, but I am at a loss on what to do next.

I will make sure my next bike has a damn electric starter too.

 

[Rogue21]

The compression test showed both cylinders at 100 psi. I realize this lower than desired, but I think it was because I was using the kickstarter. I was more over looking to see how the cylinders compared to each other.

 

[retiredgentleman]

The compression test showed both cylinders at 100 psi. I realize this lower than desired, but I think it was because I was using the kickstarter. I was more over looking to see how the cylinders compared to each other.

Did you kick it 7 or 8 times? 100 psi is really bad for a new re-built engine.................either your engine needs to come apart again or your compression gauge is not working.

 

[Rogue21]

I only kicked it about 4 to maybe 5 times per side. I can have a go again tomorrow.

I am not sure how much of a factor this is, but I only weigh in at about 150 lb and I have 20w-50 oil in the case and I am working in temps around 30 degrees F at best. I'm givin' her hell but I'm no expert at this!

 

[Rogue21]

*Double Post*

 

[retiredgentleman]

I only kicked it about 4 to maybe 5 times per side. I can have a go again tomorrow.

I am not sure how much of a factor this is, but I only weigh in at about 150 lb and I have 20w-50 oil in the case and I am working in temps around 30 degrees F at best. I'm givin' her hell but I'm no expert at this!

Did you hold the throttle wide open?

 

[5twins]

Hold the throttle wide open and keep kicking until the gauge stops rising, however many kicks that takes. Results are usually a bit better on a warm motor but it seems you don't really have that option.

You said you switched plug wires around but the right cylinder still didn't fire. However, this indicates that the previously right plug wire was now firing the left cylinder for you. That would seem to indicate the ignition is OK and firing both plugs, but neither wire will fire the plug in the right cylinder. Maybe that plug is bad or terribly fouled, try a new one. You may also have something off with that cylinder's valve settings or carb. Is the right plug wet with fuel?

 

[Rogue21]

Did you hold the throttle wide open?

Throttle was held open during the compression test, but I will have to try again in with more kicks.

You said you switched plug wires around but the right cylinder still didn't fire. However, this indicates that the previously right plug wire was now firing the left cylinder for you. That would seem to indicate the ignition is OK and firing both plugs, but neither wire will fire the plug in the right cylinder. Maybe that plug is bad or terribly fouled, try a new one. You may also have something off with that cylinder's valve settings or carb. Is the right plug wet with fuel?

This crossed my mind tonight as well. The spark plugs were purchased about 2.5 years ago but never used. I will buy a new set tomorrow and see if I have any better luck. The spark plug in the cylinder that would not fire (right) was wet with fuel when I pulled it out for inspection. I am also going to reset the cam chain and check valves as well. If none of the above works, next step will be to look at the carbs.

Thanks again for the support!

 

[Bandit]

There are different styles of floats one is set at 22mm and the other is set at 24mm. I would check to see which style you have.

 

[5twins]

If they are '78-'79 BS38s, they would have brass floats and the setting would be 24mm ± 1mm. Here's a chart I made up showing all the particulars for the various stock carb sets .....

The usual routine for basic mods (pods and pipes) is one or two up on the pilots and one to three up on the mains. The exception is the '76-'77 carb set. The 122.5 mains it came with are so small you may need to go up 4 or 5 sizes. Also, with the larger mains, you usually need to lean the needles a step.

 

[Rogue21]

There are different styles of floats one is set at 22mm and the other is set at 24mm. I would check to see which style you have.

I do have the brass floats, but its always good to double check what equipment you are running and verify the correct numbers.

I put new plugs in today and still had no more success. I started looking closer at the right side and I think my big issue is the right exhaust pipe being completely (mostly) plugged with rust!

I got the exhaust from the PO, and well we all know how that goes. I don't have a backup set so I am dead in the water for right now.

Does anyone think I could have potentially blown any gaskets or seals due to a plugged exhaust? I'm worried I could have damaged something..

 

[Rogue21]

Alright, I just performed another compression test and was able to hit 130 psi in both cylinders. This still isn't as high as I would like, but I was being performed on a cold engine.

I don't know how I never noticed this before, but when installing the right cylinder spark plug it felt loose. The threads per inch are the same because the plug does not bind, but the diameter is off. It looks like the spark plug hole was damaged and maybe someone drilled and taped a new thread??

When I rebuilt the engine, there were grind marks on two of the ports. I could never figure out the what or the why with those marks, but I know that means the engine has been apart before, and it could have been to remedy this spark plug thread problem.

I can hear air (compression) escaping from the poorly seated spark plug threads. I guess I have two options:

1. Try to figure out the diameter and somehow find a new plug that fits said diameter and pitch.

2. Find a replacement head and do a top end rebuild.

This project has been 1 step forward and 0.9 steps backwards.