Header pipes: hot and cold?

I have a 1979 650SR. The bike has 11,000 original miles. I am refurbishing it. The carburetors have been completely rebuilt after a trip through the parts washer. The petcock is rebuilt. The timing, etc. is all re-set to spec on the engine.

The bike fires right up and idles nicely, revs, holds idle, etc. BUT ... the #3 and #4 header pipes will boil water from a spray bottle if misted after just a few minutes of running. The #1 and #2 header pipes stay very cool to the touch, won't turn water to steam.

Am I chasing something with the ignition here? Or fuel related? Or more?

The bike has a 4-into-1 megaphone exhaust on it, not the stock pipes.

Hi barquirt, here's some suggestions:

this is just on cold starts? do you ride it yet? After you ride and heat it all up for a while, they are closer? how are the plugs looking?

Ignition doesn't pair 1 with 2, so it should be less likely.

are the hot ones hot due to cylinders running lean?
- use a piece of rubber hose to listen for leaks where the pipes meet the exhaust ports
- use the wd-40 to check for leaks on the intake rubbers

Float levels proper?

Set idle mixture and vacuum-sync the carbs? check it again..

I haven't had an extended ride on it yet so I do not know if the #1 and #2 headers will heat to match #3 and #4 with time.

It starts fine with the choke, can switch off choke after just a few minutes and the bike idles nicely.

Intake boots are new.

Carb floats are new (part of the kit). All the metal tangs look to be of the same height (even with the brass bar the float pin pivots on). Needles, seats, etc. also new.

I bench-shimmed the carbs using a piece of wire and set all four slide heights by it so they should be pretty close to matched.

Idle mixture ... all the air screws are 1 3/4 turns out from seated.

Plugs: after running the plugs look almost new. No deposits or soot or other bad colors.

barquirt wrote:

The #1 and #2 header pipes stay very cool to the touch, won't turn water to steam...Am I chasing something with the ignition here? quote]

I wouldn't think it's ignition because 1 & 2 are on different points and different coils. Ed

One thing you could do to see if #1 and #2 are firing is pull off the plug cap (with something insulated or plastic) while running. I'd also check to make sure you have good spark and ensure the valves are adjusted properly.

If it's a carb issue, you may need to tinker with the idle mixture a little bit. Supposidly you can hear a small increase in idle when you find the "sweet spot". Otherwise maybe you have a partial blockage in some of the pilot circuit passages?

Also, you should do the fuel service level method instead of just looking at the floats themselves. I found that the fuel service level can vary widley from carb to carb, even though the floats and tanks all looked identical.

Set fuel levels with clear tube and then sync the throttle plates.

Check the compression to make sure those weak cylinders are not sick for other reasons?

bountyhunter wrote:

Set fuel levels with clear tube and then sync the throttle plates.

Check the compression to make sure those weak cylinders are not sick for other reasons?

+1!

Deffinately check the valves as mentioned.

If 3 & 4 cylinders are getting hot, it is not a spark issue. The spark travels through both 1 & 4 plugs, and 2 & 3 plugs at the same time on both compression, and exhaust strokes. If you have bad spark on one side, then you have bad spark on the other, and this is not the case.

Make certain compression is good and within about 10% similar across the board.

Set fuel levels identical as possible. High or low spec is not a big issue, but same height is most critical.

Perform a proper bench sync before re-installing the carbs. Afterwards, hold the carbs up to a light and make sure that as you open the throttle, it appears to have the same crack of light coming through all four barrels.

Good Luck!

barquirt wrote:

Carb floats are new (part of the kit). All the metal tangs look to be of the same height (even with the brass bar the float pin pivots on). Needles, seats, etc. also new.

Idle mixture ... all the air screws are 1 3/4 turns out from seated.

You definitely can't judge fuel height in each carb by looking at tangs on floats, as almost all seat differently. You need to at least measure their drop, and height too closed at factory specs, or go with the clear tube method.

Same with air mixture, if 1 3/4 is the stock starting point, it is just that a (starting point), that will normally allow the bike to run so it can be fine tuned from there. You will need to adjust each carb to it's own liking. If you don't have gauges you should be able to get close by feeling for an increase/decrease in heat at exhaust, and hear an increase/decrease in idle speed.

I thank you all for your input.

I don't think the cylinders are shot ... the bike doesn't smoke and when I put the finger over the spark plug hole and crank i get plenty of "push".

Valve clearnace? A thought but with so few miles it seems unlikely unless I'm unlucky.

I am suspect of the carburetor balance and fuel level height. I will research the clear tube method (I have two manuals on this bike, the Kawasaki factory book and a Clymer, see what they say) and I do have gauges to run the balance test.

Progress will be posted (if any!)

barquirt wrote:

I don't think the cylinders are shot ... the bike doesn't smoke and when I put the finger over the spark plug hole and crank i get plenty of "push".

Using finger method of testing compression is ok for determining if engine will run, but not for cylinder condition. If you push your finger down as hard as you can and the compression pops your finger off hard, then its over 100 for sure and will likely run fine. It doesnt mean the cylinders are close in relation to eachother at compressions higher, which is more important.

Valve clearnace? A thought but with so few miles it seems unlikely unless I'm unlucky.

Valve clearances are to be checked and adjusted if neccessary every 5000 I believe. Observe the maintenence procedures and time intervals in your manual and perform all of them!

If you can hold your finger down on the plug hole and the engine doesnt pop it off when pressing your hardest, then that cylinder is shot.

Good Luck.

barquirt wrote:

...bike fires right up and idles nicely...#3 and #4 header pipes will boil water from a spray bottle if misted after just a few minutes of running. The #1 and #2 header pipes stay very cool to the touch, won't turn water to steam....

:huh: Doesn't make sense that engine would idle "nicely" when only combusting on two cylinders.

Good Fortune!

Here's where the Colortune product would come in nicely for trouble shooting....

Being able to see inside the chamber during firing, you'd be able to SEE the firing condition, too lean, too rich, not at all, working OK at idle but not throttled to mid range etc....

As mentioned...
Plug wires/caps good?
Plug condition/comparisons?