Not what I was expecting on the first start of a new engine rebuild...Advise?

[Skyguyz]

Kinda-Noob Disclaimer: This issue could be and prolly is due to operator assembly. I was hesitant to post this due to the answer being something that was done really wrong on my part. I took every precaution in the engine build and no corners were cut.

The engine is a fresh complete rebuild, using OEM parts, new everything almost.

As I went to start it for the first time, it was only firing on the center cylinder. I stopped the starting process andtiming and spark strength, etc. After adjusting the timing I noticed left and right firing opposite each other. I then checked the found the the previous owner had the left and right coil wires connected at the coils wrong, (*The white wire into the coil was hooked up to the right coil and the black wire to the coil was hooked up to the left coil.) Really, who would hard wire the 2 coils incorrectly??? I corrected this issue.

I got the bike started but there is something strange going on. It runs rough and with the air filter box off, I can feel some air coming back out the left and right carbs. The center carb sucks air good and the exhaust temperature is higher on the center exhaust header ,(not much but is warmer) at the head. The spark was found to be good on all 3 and the timing is spot on with good spark, (I used a dial gauge in the spark plug holes and and verified exact timing for all 3 cylinders. The pistons and rings are new, (Wiesco's). So.....

I am sure the pistons have been installed correctly with all 3 top pistons arrows pointing forward. I suspect the crank seals and will probably do a leak down test but the crank was re-done by Bill Bune engineering and has a great reputation for crank rebuilding specializing in Suzuki GT750's. The end seals on there crank are new and I made doubly sure they were in the groves before I joined the case halves. The middle crank seals, (New) were positioned exactly as per the Baz rebuild DVD.

The bike needs a shot of starter spray to get running with the choke on and will run with the choke off but it is defiantly not right and runs rough. (*Will rev up and down with a twist of the throttle but is defiantly very, very sluggish in response to throttle input.

*I have a 74 kettle I just acquired and switched the carbs, The issue is the same regardless of which set of carbs on engine.

I did a search of the reasons air is being sucked AND blown back through a carb intake and didn't find many good answers.

Why are the left and right cylinders pushing air back through the carbs and also sucking enough air to run, but run rough?

I fear the worst....Gulp!

Thoughts? Opinions?

[Alan H]

Take the complete coil set off and mount it the other way. Left will be right and right will be left.
Or actually, that's how it is now, so left and right will go where they should. Make sure the timing is correct too, that can affect blowback slightly.
Just a thought, left and centre pistons are the same, right is different. You did fit them as they should be?

[Skyguyz]

Take the complete coil set off and mount it the other way. Left will be right and right will be left.
Or actually, that's how it is now, so left and right will go where they should. Make sure the timing is correct too, that can affect blowback slightly.
Just a thought, left and centre pistons are the same, right is different. You did fit them as they should be?

Alan,

Thanks for the quick reply as always. Another point of view is usually what triggers the right answer. I did fix the coil wires by cutting and resoldering them to the correct coils. I used the correct gauge wire and shrink tube to do this. I Reviewed the video I took and it shows the correct 1-2-3 pistons in the correct holes as well as the arrow facing forward on the piston tops. Keep your thoughts coming, it always helps.

PS: I know the right piston is the odd one but I can't remember the difference, Anyone?

Cheers,

Sky

[Alan H]

Look at the exhaust ports. The right one goes right, the others go left. You can compare exhaust port direction to piston cutaway angles.

[Skyguyz]

Look at the exhaust ports. The right one goes right, the others go left. You can compare exhaust port direction to piston cutaway angles.

Alan,

I Will take off the headers and take a peak to confirm. When I had the cylinders bored and honed, I made it clear that there was two lefts and one right pistons. He marked them and 1-2-3 which was the order I installed them. I should have checked the pistons themselves for the cutaway angles prior to the installation. Wiseco had the piston boxes marked L. L, R and I dropped off the pistons with the cylinders at the machine shop. With the pistons marked 1-2-3, it should be a no-brainer but assuming they were marked correctly without checking the cutaways could be the answer. If it is then it is certainly a blunder on me for trusting the markings the machine shop did.

I will report my findings.


Cheers,

Sky

[Skyguyz]

Well as expected, the left piston was in the right cylinder and the right was in the left cylinder. I feel a bit dumb but....

When I dropped the cylinders AND pistons off at the machine shop I explained there was a left, right & center. They understood and when I picked them up the pistons were marked with a sharpie on top as 1, 2, & 3. I had asked that these were left to right and they said of course. Without me checking the cutaways on them, I installed them.

I was able to swap the left and right without removing the engine from the frame. It was a very delicate procedure but it was done without any issues. the cylinders went back down without a hitch, (The hardest part was keeping the rings on their respective pins) and the cylinders seated just fine. I will button up the rest tomorrow and should be running by then. The difference in bore from left vs right was 2 thousands and that is not enough to be concerned.

I was so carful at every stage and feel dumb that I didn't catch the issue. I will press on finish this build finally.


Cheers,


Sky

[Alan H]

Oh dear, the correct engineering term for things like that is 'oh, botheration' or something else maybe, depending on religion, patience and bad langage knowledge.
I have lots of the latter, having had nearly 50 years of varied engineering knowledge before I retired!
Anyway, back to the issue at hand - nothing has happened that can't be sorted with a re-think and patience - it's not life threatening.
I would recommend watching this DVD (ebay number 264266598754).
Baz builds GT750s (and other Suzukis) for a living and is very knowledgeable on them. Have a look at the other stuff he does too, something else may be useful during the build. Even looking at the item pictures can be handy if you're unsure about how something goes together.
Most of us have done something 'daft' that later is obviously wrong - the infamous 'doh' moment when we realised. We all have 20-20 hindsight. Keep on keeping on and don't get despondent, it'll come right.

[malky]

Lucky you didn't get one of the rings snagged in the exhaust port - when I first had my kettle someone had mixed the piston up and the left one snagged the bottom ring and scored hell out of the barrel strange thing was the bike still went well that wasn't the reason it was stripped down

[Skyguyz]

I was able to correct the piston swap without removing the engine from the Frame. A bit of a delicate procedure and no damage resulted. The bores and pistons are as new.
The bike started just fine and ran smooth. It now starts on the first tick with no issues. I have some small issues with a switch and a headlight wire and a few other small
items but I rode it around the block and the clutch, brakes and transmission work perfect. I will post some pics and video on the restoration thread soon.
Special thanks to Alan who's Engineering prowess helped guide me through this build. Not knowing this machine has been a learning curve for sure.


Cheers,


Sky.

[Alan H]

Glad it was a fairly straightforward fix. What's next?