Bogs a bit when hot

[griggio]

Hi, my first post here... Got a 1973 T500. Changed all jets in the carb and it runs smooth enough. Only problem is, it performs a lot better when engine is cold , as the engine warms up, performance decreases somewhat, less torque. Any ideas anyone?. Compression is 135 and 120 and piston slap has been there for awhile. Blowby easier when hot? I dunno. Waiting for your comments. Thanks...

[titan performance]

Are you running the standard airbox and jetting?

[griggio]

Titan Performance,
Yep, standard airbox and jetting is correct on this bike. spark plugs look like they burn a bit rich but i have the b8hs on them which are colder than stock B77.
Any suggestions on jetting improvement?
It,s especially the low end torque that suffers a bit when hot. Bogs between 2-3000 rpm.
THanks for your input.

[tz375]

Where, may I ask, did the jets come form - Suzuki/mikuni or a jet kit? If genuine jets, try to lean out the idle slightly and if non genuine, replace the jets with Mikuni jets

[titan performance]

As TZ suggests, some of the after market carb components have a reputation for being a bit dodgey.....
Is your air filter nice and clean, and unobstructed?

[Pete O'Dell]

Sticking choke valve? and the comp difference is too much, was the comp test with open throttle?

checked timing?

Correct barrels? I ask that as thats a common problem with GT barrels plonked on early T500

[griggio]

Thanks for all your replies.

This bike really performs when cold, but 5 minutes later, it's a lot less torquy at pretty much any rpm. a bit discougaging.

I backed the idle screw half a turn from the 1 turn open position and got slight improvement on low end torque but also got bad idle . My jets are supposed to be genine Mikuni having the double square symbol marqued on them .
Airbox is original and clean and new foams are in place, still not oiled up. Timing is right on, ohmeter needle moves on marks...

I was thinking of lowering the needle another notch (it's now on the 2nd) and read the plugs, or maybe even changing needle jets from P4 to the next larger (P3 or P5). Too radical? I just want results and a fun bike that reacts. without burning pistons that is...

Any ideas or experiences would help... thanks.

[Suzukidave]

Have you done a compression test ?

[tz375]

For the latest information about idle and pilot jets, it sounds like it is rich just off idle. That is either worn needle jets/needles or a wrong throttle slide.

Inspect both pairs under a large well lit magnifying glass. Most likely to be an oval needle jet.

It could also be a blocked air jet passage.

[titan performance]

It's always tricky diagnosing on the net.... but personally, I have never gained anything by running the needle anywhere other than in the middle.

[griggio]

Thanks for all the info, guys. Well, it,s getting a bit cold for cruising around here ( 4 deg C) this morning.
So, snowmobiling is here soon. Any of you guys switching to old snow machines yearly?

i did a compression test again and it's 125-130 both. Just wondering though if float height is very critical on those T's...

How about your machines? Reacting the same way?

I had the same problem with my gt 750 - I mean, that thing had at least 30% more guts and power when cold but gradually lost quite a few horses after warm-up.

[tz375]

It's true that motors seem to make more power and be crisper when cold - ask a drag racer about that. They fire them up at the last moment. And on the dyno it's not unusual to see a power drop as the motor heat soaks.

An yes, float level can make all the difference.

[Vintageman]

griggio;

I have a few older snowmobiles not as many anymore (hand 11 nice running late 70s – early 80s Ski Doo/ Moto-Skis) for a few years ago I switch to old bikes….

To your question. Yes float height matters and is sensitive and if that different then specified all other metering components will be affected. Also make sure all your parts are set and like new. For example you can get 159 series Needle jet in place of obsolete 188 series if you needed to.

Piston slap? Are you sure you are not pinging at times instead. Piston slap is there all the time (Idle etc.) Ping sound may appear only certain times when running high RPM with low throttle opening or hard torque pull at wide throttle for examples. Usually too lean or choosing to run a bit rich can help when other factors are involved. They can sound similar…

"Bog at 2-3000 RPM". One sure possibility is your ignition system is week. My experience is when ignition is week it manifest itself at low RPMs more so first.

Run correct heat range/gapped plugs. Run Iridiums (can help week ignition, but only a Band Aid if there is a real issue). Fresh Points (look for a deep pit in your points (that won't sand file off easy). Check resistance of you plug caps. There may be a resistor in there that fails. Simply change the cap if originals or old. Check resistance of you ignition coil.

I recently again wasted a lot of time jetting when I had a bad aftermarket condenser pit new points.
Also, if your coil is week (internal insulation failing) it can get worse once it heats up due to current flow through it.

Sometimes if you’re lucky you’ll get the subtle bang or pop backfire out of exhaust letting you know there is an issue with ignition, but not always if marginal and why it can feel like jetting is off too.