rebuilt engine low compression top speed!! worn cam

[cornishgs]

Any Ideas for my compression figures for my 79 gs550 will get to 70 quite quick but just runs ou of steam at 80mph???


comp hot 135 130 135 135

comp cold 125 120 125 125

wet test cold 150 145 150 150

checked with throttle wide open good battery
have fitted o/s pistons, new rings cylenders rebored and honed ,valve lapped and checked .valve timing checked, valve clearance checked, carbs bench synced

done approx. 700miles since engine rebuild, have run it gentle and hard to try to bed rings in.have run on 15 40 cheap diesel mineral oil for last 200 miles

runs fine dosent smoke ticks over good,

intake cam lobes were worn down just below service limi on intake lobes I have got new used cam to try lobes like new havnt fitted yet.


bike has standard airbox carbs jetting motad 4into 1 (requires no jetting change) plugsall look good light brown

ignition timing checked and advance checked fuel tap flow checked

it runs good but just seems to run out of power at 75 80mph no misfiring should I just continue to ride it and see what happens any suggestions

[Nessism]

The compression seems okay. You might want to check cam timing again, and make sure the spark advance is working smoothly. Not sure what else it could be other than maybe you are loosing spark in one cylinder.

[JTGS850GL]

You know the intake cam is out of service limit and you have a replacement cam. I'd start there.

[Nessism]

A 550 should easily top 100 mph. Struggling to reach 80 sounds like it's only running on three cylinders. Low cam lift won't cause that much power loss.

[cornishgs]

Thanks for suggestions

it gets to about 7000 8000 rpm in 5th and 6th and just runs out of power ,wont have chance until this weekend to check.

will double check cam timing again.
and ignition timing first.

If cam timing is not out, I will pop in cam as well to rule cam valve lift out.

its had 1 recent new coil, plus new points condensors. maybe other coil??

[cornishgs]

so maybe spark related ,
was worried compression and hadn't thought it might be sparks

[robbiem1961]

Have you taken the carbs off and cleaned them, a damaged or incorrectly set float will cause you problems, as will any worn or damaged jet, needle, slide etc; on my old gs 450 T she ran brilliantly up to about 75mphwhen she would start mis-behaving, pinking (preignition) lacking power and the usual unhappy motorcycle symptoms, the issue with her was a miniscule split in one diaphragm: after the carbs were overhauled that little motorcycle ran sweet as a nut
I hope this may prove to be of some use to you good sir

[chef1366]

Compression is good and will get better over time as the rings seat.
I would make sure the main jet you are running is not too lean.

[cornishgs]

yes carbs have all been totally stripped new orings etc

sonic cleaned etc

[cornishgs]

I have rechecked cam timing its spot on at tdc arrows line up with rivet pins 20 links etc.
whilst there I will replace worn intake cam 2 lobes worn below spec and reshim after if needed.

I have taken carbs off to get at tensioner so I will take carb bowls off and fit 82.5 main jets, 80 fitted at moment and check float heights, again maybe raise needle a notch to richen?? motad exhaust says no jetting changes required.
Motad don't claim any power increase whith exhaust it is more of a replacement system with similar power band flow as original. I will hopefully get somewhere soon.

Ihave ordered a new coil and leads £20,00 new off ebay just to rule out any funny goings on with remaining coil as already replaced rhcoil 3mths ago

[robbiem1961]

Good morning my mate, i would fit new needles/jets, they may look new but if worn they're of no use, stay with the sizes and needle groove positions specified by suzuki, you will need to ensure that there are no air leaks, fit new rubber inlet manifold boots and 'o'rings, also airbox to carb hoses this 'WILL' make refitting of the carbs a billion times easier, check the air cleaner filter is not blocked or damaged, or just replace that as well, and that the airbox body is good and useable; again i will reiterate inspect the carb diaphrams, replace if you have the slightest doubt, ask your local suzuki dealer mechanic to inspect them...they are so expensive i 'WANTED' my 450 ones to be useable, but he condemmed one,so i replaced them as a pair..
without ensuring that everything else is as it sbould be, air tight, leak free, changing jet sizes, needle positions etc is (an attempt at) compensating for the wear and breaking down of various components, without resolving these first you'll be grabbing at straws and pulling your hair out;
I'm hoping that it allgoes well for you my mate

[cornishgs]

H fellow Brit, have gone through carbs earlier in year.all look good,

they don't have diaphams luckily as they are vm slide carbs pre 1980

carb manifold intakes with new o rings are like new.air filter new .air box all good.

I will get there in the end just need to check through everything again fitted my better used cam last night and need to wait for 2.65 shim to arrive £5.00 off e bay all clearance are .06 to .08mm now. would be nice if I could get it over 80mph one day whooh

[robbiem1961]

hmmmm? i've not had a huge amount to do with the old slide carbs, i did have a 1983 T140 Bonneville but they were amal (crapalls) slides, we fitted new needles, jets; but they were quietly and unassumingly knackered; the bodies and slides were worn and the pair were relaced with new
There's a carburetor refurbishment company in Manchester called NRP Carburetors who re-manufactured my obsolete diaphragms; they are very much 'on the ball' with regards to old school japanese motorcycle carbs; it's what they do; and will willingly discuss any possible issues or common problems and how to cure them;
i wonder whether i the advance/retard mechanism is in need of some TLC, cleaning and a tad of oil; or replacing if she's worn and allowing the timing to wander; I'd go electronic; fit and forget;
more major work next; is there a weak or broken valve spring; the only time my mate Garys' 550 L stopped on him was when two up; coming home from wales and going as fast as she could she bounced a valve;
motorcycles... who'd have them
here's a thought, is the worn cam lobe indicative of lubrication failure, if so is the cam-cap worn?

[hampshirehog]

Like you said, the Motad won't require any rejecting.

A few other (unlikely) suggestions:

Is the exhaust clear (no dropped plates or cr@p stuck in there)?

Is the fuel tank (via the filler cap) venting ok?

Are the inlet rubbers in good condition?

[cornishgs]

Like you said, the Motad won't require any rejecting.

A few other (unlikely) suggestions:

Is the exhaust clear (no dropped plates or cr@p stuck in there)?

Is the fuel tank (via the filler cap) venting ok?

Are the inlet rubbers in good condition?

motad is new stainless 2months old,

tried a ride with fuel cap loose no difference

,inlet rubbers are like new,sprayed carb cleaner around to check for leaks before .so where I,m at at moment checked

cam timing spot on
put better a better used inlet cam in like new
valve clearances checked and adjusted again. waiting for a 2.65 shim so can put back together all cam bearings were checked with plastigauge all about.050 clearance or less not scored at all

Don't really think it would have been cam wear causing it to run this slow but whilst I had a lookin there it seemed a good time to fit it something else to rule out.

throttle slides all opening
I have gone up 1 size on main jet to 82.5 from standard 80 just to try whilst carbs were off
will stick back together when shim comes and give it a run.

will fit shim and stick back together and give it a test next week hopefully . all good fun!!