What causes an engine to over-run?

MikeH1A · Post by **MikeH1A** » Sun Sep 27, 2009 4:56 pm

Our Classic Jap group went for a big run yesterday. The T500 performed pretty darn well - running around 70mph for most of the ride.

What I noticed however was that when I began to close the throttle, the engine started to lurch a bit and get that jerky performance like when you go down hill on a two stroke. Once I started to bring the throttle back on it settled - only to repeat the same thing when I slowed later on.

Any thoughts what is happening here

Overall, I'm pretty pleased with the old girl and she's done about 1500 miles since I recommissioned her.

Thanks

Post by **Suzsmokeyallan** » Sun Sep 27, 2009 5:38 pm

Mike what you are describing sounds like a slightly lean condition on a part, or parts of the carb circuit.
Is the engine stock with stock filter and jetting.

MikeH1A · Post by **MikeH1A** » Sun Sep 27, 2009 6:29 pm

Yes, she's stock in terms of airfilter and carbs.

The one thing I just thought of is that we did climb quite high (from sea level almost up to the snow - even though it's meant to be spring here

). Could that impact on the performance but then wouldn't it level out on the run home as we got back to the usual level?

Thanks for your thoughts, Mike

Post by **Suzsmokeyallan** » Sun Sep 27, 2009 9:26 pm

Carbs are touchy to altitude, for example my buffalo in Calgary at about 3,500 ft runs perfectly with the carbs set at 1 1/2 turns out on the mixture screws, but it needs to go to 3/4 to 1 turn out when it hits sea level in Vancouver.
What i should have also asked you was are the float levels at the stock setting??. The engine should run correctly if its jetted for a certain altitude even if you leave it and return to that altitude, but theres obviously something not quite right on your bike so you'll have to do a bit of snooping around to find why its doing that even after you returned home.
Have you rechecked the intake tubes?? you might have a possible slack intake connector flange etc and now a minor air leak.

Craig380 · Post by **Craig380** » Mon Sep 28, 2009 2:24 am

It's also worth checking the oiling on the foam air filter element, a couple years ago I cured a persistent mid-range stutter / misfire on my 380 simply by cleaning the air filter and re-oiling it, using more oil than I had previously ....

the ideal seems to be enough oil to leave a light trace on clean hands, but not so much that it's soggy and dripping ...

tz375 · Post by **tz375** » Mon Sep 28, 2009 8:01 am

Large bore two strokes have a propensity to surge on a closed throttle. Funnily enough, it can be either lean or rich.

If it occurs only at higher altitude, it would be because it is rich when the air density drops.

On a 500 you could try opening the air screws another 1/2 turn at altitude to lean it out slightly, but remember to adjust them back when you go down the hill.

MikeH1A · Post by **MikeH1A** » Mon Oct 12, 2009 3:09 am

Just had the old girl out today, riding from sea level to 200 metres (so not high today). She performed quite well, accelerating nicely when around 4k and above, not gulping fuel or oil but still some over-run. So, I think I've eliminated the altitude thing as a red herring and will check out your other suggestions.

Is there anywhere that has a complete list of the appropriate size jets and everything for the 71 T500? I'm thinking back to when I rebuilt her and really just used what the PO had in the carbs. Based on some of his dopey behaviours with other parts of the bike, I'm wondering if the carbs are a mishmash of pieces. I do have to say that she's not running terrible or anything.

Thanks again Mike

Post by **Suzsmokeyallan** » Mon Oct 12, 2009 5:57 am

Mike did you look at the parts listing for the 71 Titans carbs, that would give you all the jets stamped info on what they should be.

Buffalo-guy · Post by **Buffalo-guy** » Mon Oct 12, 2009 9:24 am

My little book of knowledge puts a 1971 T 500 with 150 main jet, #30 pilot jet, P-5 needle jet (possibly P-4), 5FP8-3 needle, air screw 1 1/4 turns out, and float height at 27.25mm. This is from Roy Bacons book on Suzuki two strokes, and I hope it is accurate. Cheers.
Fred

H2RICK · Post by **H2RICK** » Mon Oct 12, 2009 4:04 pm

Well, according to the info I have to hand, the T500R specs are what Fred posted except the needle jets are definitely P-5 on both sides. Slide cutaways should be 2.5. The carbs were identified as 15602 but those markings are probably long gone unless they metal-stamped them on. If stamped, then the stamp mark will usually/usually be on the carb body in the area of the enrichener plunger hole on the forward-facing side.
Where the confusion arises about the needle jets is that supposedly the J models, for some unkown reason, also used the identical carbs with everything the same as the R models BUT.....used a P-4 needle jet on the L/H carb and a P-5 on the R/H carb.
No, I have NO clue as to why the factory would have done such a thing.
So there's another piece of semi-useful Suzuki stroker trivia for your memory banks. You're welcome, I'm sure.

MikeH1A · Post by **MikeH1A** » Mon Oct 12, 2009 11:10 pm

Many thanks for all the info guys. I've done about 1500 miles since the recommissioning, so it's probably time to go thru and check/tighten everything. I think I'll pull the carbs and check all these small but important bits plus check for air leaks as suggested.

One other question: My points will need redoing, has anyone had experience with the Sudco points and condeser kits? Are they ok to use or stick to genuine Suzuki, if available?

Thanks, Mike

Craig380 · Post by **Craig380** » Tue Oct 13, 2009 1:41 am

In the UK, the most common type of aftermarket points is made by Daichi, they fit and work OK.

I seem to remember something about the fibre 'heel' on the Daichis being different length to the Denso points, but they worked fine for me.