Heat Seize Issue...

[Hughieboy]

I've done a bit of home tuning to my engine and now it keeps wanting to heat seize.... It started as a Harry Barlow 190 running a 22mm Jetex and big bore exhaust. I have added a Varitronic, Dellorto PHBH 30 which was supplied with 'ball park' jetting for a Mugello and a TSR EVO expansion. I've set the timing up to 19 degrees as recommended by the Cam Lam website and have matched and polished the exhaust and inlet ports.

It starts first time, is much more responsive than before as well as a bit keener through the Rev range. Trouble is, if you try to open it up for any length of time it starts to nip up and so,it's grab the clutch time. It never used to heat seize at all with the old set up and you could ride it flat out everywhere no,proble, albeit less lively than it is now.

I'm a bit green at this and not sure where to start, is this a jetting issue? Only other symptoms are that it hits a flat spot somewhere after 5000 revs ( not sure where, the eBay rev counter I've fitted seems to indicate a drop to 3750 revs after 5000 passes!) and it does seem to miss a beat occasionally, not a splutter more an occasional intermittent misfire.

I'd be grateful if this sounds familiar to anyone?

Thanks.

[dan66]

I would recommend x7 needle 40slide
Start with 55 pilot ant 118/120 main
Check your timing
If your runing advanced ignition make sure it retards to at least
15/14 degrees at full throttle ie 6500 rpm using a strobe light

[MickYork]

Not 100% on the varitronic but I think it retards about 8 degree. If you're setting it at 19 degree then when your hitting high revs you will be retarding to nearly 11 degrees........very retarded as most run at 16/17. i'd check what it's retarding to with a strobe.

That said it shouldn't cause a heat seize. The main thing to check is the piston to bore gap. Too tight a fit and that's your problem.

Fuel starvation could also be a cause so It may be worth checking your flow rate from the tank and check you have a big enough float valve fitted in the carb'.

[dscscotty]

I would also make sure you have at least a 300 fuel / needle valve fitted, you may be getting fuel starvation, is there any pinking (Detonation)?

[Hughieboy]

No pinking and it can't be the bore size I don't think..... I've done a good couple of thousand seize free miles on it prior to changing the carb and exhaust and fitting the Varitronic...... I'll need to double check it all, I just wondered if the symptoms might be typical of x, y or z so I had an idea where to look first as not sure if it will be timing or fuel..... I guess this is what happens when you change everything at once.....

[johnnyXS]

unusual for an engine to try and seize when accelerating ? more usually a hot engine will try to seize when it is deaccelerating due to lack of fuel lubricant because you are running only on the pilot jet .
Are you sure it is seizing or could it simply be fuel starvation or something else?

You need to check the float setting and needle valve carefully and also disconnect the fuel line and check the fuel flow from the tank so that you can eliminate both of those first.
Clean or replace your spark plug and read the plug .....it will tell you what your fuel mixture is like

[Warkton Tornado No.1]

My apologies as JohnnyXS said pretty much what I am about to post, but Hey! Great minds & all that:


The problem with changing several things @ once is the elimination of issues such as you are experiencing now.

It is difficult to have a standard ‘setting’ for an ignition that retards, but for now, copy what somebody else has & that should be checked by several markings on the mag housing & re-strobing.

Checks should be made for air leaks in inlet & exhaust.

I take it you have a fresh, correctly gapped, fresh plug fitted?

Then, & your symptoms sound like this to me, check for fuel delivery. That is, cleanliness of tank, breathing of cap, flow & rate through fuel line (in running position to see if there’s a kink you’ve overlooked) & line filter if you have one fitted. Finally the carburettor: filter, needle valve, float needle, float & its tabs, breathers, choke (for leaks), jets cleanliness & swap if need be for alternatives, needle.

Good Luck!

[Hughieboy]

Thanks, I appreciate you all taking the time so just a couple of points....

Tank is new and cleaned through, cam lam fast flow tap working fine and fuel getting through well. I did a leak down test using the inner tube and pump method prior to fitting the carb and exhaust and found no leaks, I had just refitted the top end, new gasket and sealant etc..

I know the fuel delivery is fine as I tested it as after fitting the carb and exhaust as I had problems that I thought were fuel starvation which turned out to be an ignition fault hence fitting the Varitronic. It has a new plug and cap which I have checked. On plug chops the colour is a chocolate brown colour and no oiling up.

Although I've had scooters for years it's the first time I've got involved in engine work to this extent though and I set the timing up so that it retards to 19 degrees when strobing given a few revs above high tick over as per cam lam website however I may have not go this right as a first timer possibly..

I phoned them and they said I should change to an alloy barrel kit but I've already dropped a load of cash on the stuff I've bought so far and the engine ran fine before so I'm hoping it's a timing or carb setting that's not correct rather than needing a whole new top end. This in itself would cause issues as it was built using a 116mm conrod with packer and Suzuki piston by Harry so fitting a kit would, I'm guessing require not just the kit but also a new crank building to suit the kit!

I suppose I could do with a better feel for whether the timing is set wrong or whether the mixture is too rich or too lean. From what I've read on forums I could be getting symptoms of all three!

The reason I think it's heat seizing is that after running at reasonably high revs for a few minutes it starts to tighten up and I believe would lock up if it wasn't for pulling the clutch in quick, coasting for a few seconds and then carrying on at a lower amount of revs. Turning on the choke as some recommend seems to make no difference to the running at this point.

If I recheck the timing, how do I know if it needs richening or leaning off? I have a spare jet kit I bought with the carb, unfortunately I'm not near anyone who has a dyno that I can think of...

[Warkton Tornado No.1]

BTW. Have you checked compression ratio?

What compelled me to write again though was what you’ve just said. Your set up to retard to 19 degrees can’t be right.

That’s a static setting for a fixed ignition so, obviously, it never advances from there.

I would have thought you want to retard fully to something less than 19, more like 16 because efficiency of the burn increases as flow rate goes up due to the surface area being that much greater (more, smaller molecules)

That’s only my opinion, but hopefully it’s a far safer ‘test’ to carry out & others might even agree....

[Warkton Tornado No.1]

Aha!

I've just read what Mick York said which bears out what I think.

[Hughieboy]

Update....

Thanks again for all your input fellas....

I have a spare main jet kit, I was told that the 'ball park' settings I was supplied the carb with would suit a Mugello 186 or similar, or so I thought. However, the main jet was only a 100 which I assume is too small.

I've changed it to a 120 as suggested however I don't have other spare jets, pilot etc but can get some. Just done a 7 or 8 mile circuit and no binding up (hurrah!) although it's not revving out particularly and I think further adjustment is needed. I have a 122 I could try? I don't really understand the correlation between the main and needle/slide/pilot though..... Currently it now has;

70 pilot
120 main
45 slide
X4 needle second clip position

I will whip the footboard off agin and recheck the timing. So if it's retarding at present to 19 at about half throttle is it recommended to retard it a bit more? Didn't try doing it while wringing the knackers off it because of the neighbours!

[MickYork]

70 pilot is too big , try 55

70 is usually the choke size

x4 needle seems strange x7 or x13 are the more common ones

[Hughieboy]

Oops.... The 70 is the choke jet, not the pilot.... Bloody amateurs.... :-/

[coaster]

As said above, your ignition must be set with a static timing that will allow the ignition to retards DOWN to 16 degrees so something in the range of 22-24 degrees static is needed. Mark the flywheel for static and 16 degrees and then check with a strobe to make sure that the timing retards down to the 16 degree mark. If it doesn't turn the stator anti-clockwise a fraction at a time until it does. Be prepared to file/dremel the slot in the stator in order to get it to retard enough.

With modern fuels you are asking to blow a hole in your piston with the amount of advance you are running.

[Warkton Tornado No.1]

Your Ignition timing should be corrected before you do anything else.

Honestly, it's the absolute priority.

[Hughieboy]

Thanks everyone.... I will get the timing sorted and report back. And check the pilot jet size instead of the choke! I really appreciate the advice.

Hopefully I'll get chance tomorrow, otherwise it will be Monday.

Cheers,

Hugh.

[Hughieboy]

70 pilot is too big , try 55

70 is usually the choke size

x4 needle seems strange x7 or x13 are the more common ones

Yes, funnily enough X4 needles aren't even mentioned in Cam Lam's set up guide and that's where the carb came from but it 100% is! Should I change it?

[Knowledge]

Hugh,

Have you had the top end off since the seizures? It is important to clean out any deposits left on the bore or piston following the seizure, because this reduces the size of the bore and makes another seizure more likely. Also, lubrication is reduced on a seizure mark, as the tiny grooves that hold oil on the side of the piston are eliminated, increasing the risk of another seizure.

Ho hum, so much to do, eh?

[Hughieboy]

No, I haven't had the top off again. It never completely seized, rather you could feel it was about to as it started to tighten up. I will give it a strip again once I've sorted the other stuff out as if I do it now and don't get it right and it starts to tighten up again I'm going to be having the top end off and on from here until Christmas!

After upping the main jet, although there is plenty more to check, timing etc, it would appear that fuel starvation at three quarter to full throttle was a major contributing factor...

So much to do indeed! Better than Coronation Street which is usually the alternative in our house!

[CHRIS in MARGATE]

First job. Remeasure TDC (Top dead centre) and remark with a good visible cut into the mag housing. Now set the timing as already recommended (strongly ).
I'm afraid you are going to have to rev it to 6000rpm and beyond to check the timing at that engine speed. No good assuming that it's working; you need absolute clarity and peace of mind.
Do the timing first.