GS450S One Cylinder hotter thn the other

[Guest]

Hi Guys,

Am just rebuilding a 1980 GS450S, Got it started for the first time, but noticed that the RH Cylinder/Exhaust is much hotter than the left one.

I pulled the plug cap on the hot one and started the bike, so it's getting fuel & sparks to the left side, just not sure why the temperature problem.

The engines been rebuilt, carbs have been vapour blasted and ultrasonically cleaned

Front and rear airbox boots all NEW , along with 'o' Rings, airbox fully sealed as required

Any ideas ?

[Brendan W]

Back to basics, fuel, compression, sparks.
You could have a weak coil or a bad supply or trigger wire. I would try swapping over the coils and a quick check of the primary resistance 3-4 Ohm.
Then a compression test and thrown somewhere in there a tap on the left carb bowl with a rubber mallet followed by removing the drain screw with the tap on prime and maybe even the bowl off. After rebuilds of anything with pipes things get left inside, welding rods, gloves etc etc

[Showtime]

Back to basics, fuel, compression, sparks.
You could have a weak coil or a bad supply or trigger wire. I would try swapping over the coils and a quick check of the primary resistance 3-4 Ohm.
Then a compression test and thrown somewhere in there a tap on the left carb bowl with a rubber mallet followed by removing the drain screw with the tap on prime and maybe even the bowl off. After rebuilds of anything with pipes things get left inside, welding rods, gloves etc etc

Hiya Brendan, thanks for the help on this.. will try a few (or all) of your ideas.. not sure how to do a check on primary resistance as wiring not by strongest point, I do have a volt meter tho.. so any further guidance on this appreciated.

I checked the spark today, seems good on both plugs, and I did the shims by the book when I rebuilt the top end.. I lent my compression tester to a friend about a year ago.. not seen it since !! will have to get a new one I think

Found the gloves !! so we are good on that one

[Brendan W]

To check the primary resistance set the meter to the Ohm scale that covers 0-10 or 0-20 and check across the supply and trigger wires. Both should be the same-ish. Failure is uncommon.
If you can see sparks of about the same strength move on to something else for a bit anyway. While you are at it remove the caps at the end of the HT leads and check for corrosion of the core. Chop off a bit to get clean core if you see any green stuff.
You can get an idea of a serious compression difference by pulling the plugs and pressing a rag against the hole. Don't let the compressed air see your skin. A big difference would be obvious.
What colour are the plugs ?
From where you are now I would check that the float bowls are free of water and grit and flush the float needle by running on prime with the drain open.

[Showtime]

To check the primary resistance set the meter to the Ohm scale that covers 0-10 or 0-20 and check across the supply and trigger wires. Both should be the same-ish. Failure is uncommon.
If you can see sparks of about the same strength move on to something else for a bit anyway. While you are at it remove the caps at the end of the HT leads and check for corrosion of the core. Chop off a bit to get clean core if you see any green stuff.
You can get an idea of a serious compression difference by pulling the plugs and pressing a rag against the hole. Don't let the compressed air see your skin. A big difference would be obvious.
What colour are the plugs ?
From where you are now I would check that the float bowls are free of water and grit and flush the float needle by running on prime with the drain open.

Thanks Brendan, Ok ignorance here.. which are the trigger and supply wires ? you mean the feeds to the coils yes ? (told you wiring not my thing ! but I'm getting better with it )

I'm thinking carb setup.. I posted some pics of the newly assembled carbs a good while ago on here.. they have been in a box for about a year.. so will give them a proper flush through tomorrow and report back... Oh yes.. ran it tonight, definitely popping and banging a bit so It's talking to me, I just need to understand better what it's telling me.. fascinating bikes are they not

[Brendan W]

Thanks Brendan, Ok ignorance here.. which are the trigger and supply wires ? you mean the feeds to the coils yes ? (told you wiring not my thing ! but I'm getting better with it )

I'm thinking carb setup.. I posted some pics of the newly assembled carbs a good while ago on here.. they have been in a box for about a year.. so will give them a proper flush through tomorrow and report back... Oh yes.. ran it tonight, definitely popping and banging a bit so It's talking to me, I just need to understand better what it's telling me.. fascinating bikes are they not

Fascinating yes, a few other f words have been used on occasion.
You are correct the coils generally have a supply and trigger wire or feed wires. The usual Suzuki scheme is that both supply wires are the same colour, 12 volts is 12 volts, but the trigger or fire signal wires are different colours left and right. Open the plug for the coil and check the voltage arriving at that side between the common colour wire and any ground point. The plug pins may be clean but the wires could have corroded in the plug and it's difficult to see in there.
If by popping you mean backfiring in the exhaust this is most often an intermittent spark on that side. Less common would be extremely weak mixtures.

[tkent02]

How about the easy things first, did you synch the carburetors? How did you adjust the mixture screws? Are the spark plugs mostly looking the same color or is the cold one black? Or completely white? Or are the both about the same shade of gray or tan?

A quick and easy check of synchronization, turn the idle speed up to around 2,000 or so, then pull off one spark plug at a time, the engine should idle somewhere near the same RPM with either one disconnected. If they are way off the carbs needs to be synched. A lot of guys with twins use this technique exclusively.

Is the cooler pipe stone cold, or a lot cooler by touch, or just a tiny bit cooler measured by some high tech instrumentation?

And how does the bike run going down the road? Is it misfiring at all? How about at wide open throttle? How about at half throttle or cruising? Or are you just letting it idle a minute or two and noticing one pipe is hotter?

If it's not missing or popping and the sparks look good it's probably not an ignition thing. That would be the last place I'd look unless it has an obvious ignition problem.

[Nessism]

Good response by tkent (Tom). A weak spark won't cause low power in the cylinder unless it's misfiring.

[Showtime]

Thanks Brendan, Nessium and tKent (my heroes all of you).. I don't have a vacuum gauge (yet) I bench synched them previously . Clearly I'll need to buy one to achieve correct balance.

Pilot screws set to 2.5 turns.. will adjust those to 3
will get 2 new plugs - mine are original so worth changing

The RH pipe is very hot after about 2 mins idling/light revving, and too hot to touch with bare hand, the left one you can grasp, it is hot, just nowhere near as hot.
I noticed minor backfiring last night when revving it lightly

Bike is not road legal (or roadworthy) right now, so no opportunity to test it properly...

This might sound a stupid question, but if both cylinders are firing (engine runs with either plug cap removed) I never would have thought an engine would run at different temps.. how is this possible ?

Thanks All.. Your input is brilliant and gratefully received, learning quickly here

[Showtime]

Heres the removed plugs - the RIGHT one is the RH Cylinder and is the HOT one, the Left one is the COOLER one

[Nessism]

Well, for one thing you shouldn't be running a resistor type spark plug. Get some standard B8ES's.

What you describe about being able to grab the pipe indicates you don't have proper combustion in the LH cylinder. Just off the cuff I'd suspect something is wrong with the carb on that side.

[Showtime]

New Plugs ordered today, out of interest, what should the compression be.. 120lb something like that or higher

[Nessism]

New Plugs ordered today, out of interest, what should the compression be.. 120lb something like that or higher

Yes, 120 psi or higher since you just redid the top end.

Do that ghetto sync job like tkent described. The BMW guys live by that method so it's got some strong backing in the motorcycle community.

[Doug]

Admittedly, I only gave all the posts a cursory look.

So, which cylinder do you like? The hot one or the cooler one? Do you have one of those laser temp guns or can you borrow one and tell us the temps of the two pipes?

I'm thinking carbs. Or a vacuum leak causing a leaner mixture. I'm not familiar with the carbs, I assuming they have adjustable needles and those are set to the same notches on both. Do the slides open at the same rate? Do they close properly?

You can check for vacuum leaks at the carb boots with an Oyy-acetelyne torch by introducing Oxygen only around the boots add see if the rpm changes suggesting a cracked or leaking boot.

[Showtime]

Just found that the screws that are for the vacuum gauge in the cylinder boots were loose, tightened them up then ran fuel through the left carb (used a Mityvac on the vac hose from the tap to help things along).

Left pipe getting much hotter. LH 58C RH 61C so much closer now.

will pick up a compression gauge tomorrow (annoying as I have lent mine out !) and also order some synch gauges to set properly