LCGB Forums

The ability to post messages is restricted to LCGB members. Any questions contact us at lcgbadmin@googlemail.com

Small block home tune

Need help with a tuning kit, how do you tune your scooter, which kit should I choose, and all general tuning and modifcations questions are for in here.

Re: Small block home tune

Postby dickie » Thu Jul 18, 2019 5:11 pm

sunrisemac wrote:Having had a look at your photos I think there may be a couple of points worth considering.

Although the burn colour on the head and piston crown look good, there is no squish band area on the head which seems to align with the position on the piston on the inlet side that appears to have had the seize. At the point of ignition, and just after, when the flame front is at its peak there is nothing to shield and slow the hot gases from flowing down the side of the piston towards the rings, (see the brown colouring at the top of the bore), and overheating that area of the piston which may possibly be causing problems with lubrication and clearances.

When running in IMHO it can be more detrimental to over oil the fuel mix, run at low revs and make the engine labour without much cooling being produced by the fan. Periodically give it a good few revs for a short burst then let it cool down so to get the new surfaces through a series of heat cycles to bed it all in.


Thanks, I do pretty much very revs and throttle position. I never let them labour whether run in or not, just because it's an unpleasant way to ride.

I did however definitely go too fast too soon on this one.

Not too sure what I can do about the head. I could buy a Ron moss head and machine from scratch but this is supposed to be a budget motor, so I'm not keen. You'd think that machining a 150 head for 175 would increase the squish band, but I seem to recall that I bought this as a 150 head...who knows? Just a lump of aluminium off Ebay. Could have been on on NVT for all I know!
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby dickie » Sun Jul 21, 2019 1:54 pm

Does anyone know if the SR 175 pistons have the same crown profile as Asso 175?
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby dscscotty » Sun Jul 21, 2019 2:07 pm

yes they do
dscscotty
 
Posts: 717
Joined: Fri May 30, 2014 7:42 pm

Re: Small block home tune

Postby dickie » Sun Jul 21, 2019 4:58 pm

dscscotty wrote:yes they do

Cheers scotty, I've gone 0.5mm oversize and mick abbey is gutting my clubman to make a sort of half-arsed expansion.

Try again.
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby dickie » Sat Sep 14, 2019 1:48 pm

Rebuilt now with new piston and an old expansion from mick abbey; apparently the first one he ever did for a lambretta, but that's by the by.

Goes ok, I'm running in more gently now, but I'm a long way from being a jetting expert.

I've increased the main jet to 108, and it feels boggy up top, but that's ok for now. I also lifted the needle one notch which made it unrideable so I put it back.

I've noticed that if I am at high rpm, guess 6000, and I pull the clutch and shut the throttle, the engine takes a long time to drop revs. Stays low once it's there, but takes a long time to drop if I just wait.

Is this caused by pilot jet too small? Or mixture screw too lean? Or something else altogether? I did a leak check during build and all was good at that point.

Thanks.
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby Scooterdude » Sat Sep 14, 2019 7:41 pm

Are you sure it’s not leaked at the stub joint ? If that’s ok I’d look at pilot jet/air screw, failing that worn or chattering slide, probably the least likely though.
Scooterdude
 
Posts: 674
Joined: Fri May 26, 2017 3:45 am

Re: Small block home tune

Postby dickie » Sat Sep 14, 2019 9:36 pm

Scooterdude wrote:Are you sure it’s not leaked at the stub joint ? If that’s ok I’d look at pilot jet/air screw, failing that worn or chattering slide, probably the least likely though.

Its a new pipe since the leak check, so maybe you're right. I'll take it off and reseal. Slide will be fine as it's a used carb, but I bought it new and it only did 1000 miles on its previous motor. Ewe
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby Scooterdude » Sat Sep 14, 2019 10:34 pm

dickie wrote:
Scooterdude wrote:Are you sure it’s not leaked at the stub joint ? If that’s ok I’d look at pilot jet/air screw, failing that worn or chattering slide, probably the least likely though.

Its a new pipe since the leak check, so maybe you're right. I'll take it off and reseal. Slide will be fine as it's a used carb, but I bought it new and it only did 1000 miles on its previous motor. Ewe

No don’t do that. What i do is fit the stub then using a thin piece of wood with a sheet of rubber I clamp this over the stub with a large g clamp around the cylinder THEN i do the leak test, pointless in doing it without the stub fitted otherwise how will you know the stub joint is good?
Scooterdude
 
Posts: 674
Joined: Fri May 26, 2017 3:45 am

Re: Small block home tune

Postby dickie » Fri Jul 10, 2020 3:12 pm

So, after replacing the piston and cleaning up the barrel, I've done about 500 miles with a little more sensitivity to the running in process.

As i was minded to change the carb for a PHBH28 that I have lying around, I decided to take the top end off for a nose around.

Piston looks good, but there's a mark on the barrel I can't get my head round.

It stop short of the inlet port although it's difficult to tell from the photo.
Image2020-07-10_02-55-18

The mark is coincident with the top ring peg. The ring peg does not protrude past the edge of the piston. I can't feel the mark, only see it. I think I'll fit a piston ring in the barrel and try shining a light up the back.
Image2020-07-10_02-55-45

What would cause this?
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby dickie » Fri Jul 10, 2020 7:20 pm

I've looked back at some old photos from when I melted a piston last year. This is an old mark which the blow-by at the piston gap has simply highlighted. I'd be happier if it wasn't there, but it's nothing to lose sleep over. I'll put an oversize piston in over the winter but it's my only functioning lambretta at the minute so I'll live with it.
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby dickie » Sun Sep 06, 2020 11:16 am

Eventually got a chance to get it on the dyno.

It's had a few pipes on it. First was the BGM v3 which was on when I seized it; that was had its baffles removed to stop it gassing up. It was awful to ride like this as it seemed to hit a brick wall at about 5000rpm (seat of the pants tacho) but hadn't even got into its stride by then. I got a used gori 48mm for about £100 and that is what was on for the dyno session. On reflection, i should have just used the old Mick Abbey expansion I have, but maybe that's for another day now.

Also, it was originally on a 25PHBL through a drilled airbox, but it's now got 28PHBH and breathe sweet filter.

The balck curve is where it is now after I swapped the needle for X7 as X13 was too rich and on top clip. Other than that, we made no changes.

All in all, it's been a rewarding experience. I'm sure I could have got better results just by buying a mugello186 and sterling pipe, but then I wouldn't have learnt as much, nor would I have the satisfaction of knowing I'd tuned it myself.

It's pretty nice to ride. I sat with the traffic up the A19 from Mick Abbey's to home no problem with just a bit in hand. Easy to pass HGVs. And not too bad on fuel; about 10 miles per litre.

I wonder if the tiny LI150 transfers are holding it back as the revs rise and flattening the curve (I did think it was GP150 barrel when I started but it isn't). If i get a chance over winter, I'll give Mick Abbey the barrel to widen the transfers into the barrel.

For now though, it rides nice, is reasonably quick and isn't going to blow itself up, well not immediately anyway :lol:

ImageLambretta tuned 175 dyno
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby Fast n Furious » Sun Sep 06, 2020 5:15 pm

Rich,
Cant really tell from your posted photo but that ring gap line up your cylinder looks wey bigger than the expected 10 thou width I would expect to see.
Can you confirm?
User avatar
Fast n Furious
 
Posts: 1502
Joined: Sat Nov 19, 2016 3:56 am
Location: York

Re: Small block home tune

Postby dickie » Sun Sep 06, 2020 5:36 pm

Fast n Furious wrote:Rich,
Cant really tell from your posted photo but that ring gap line up your cylinder looks wey bigger than the expected 10 thou width I would expect to see.
Can you confirm?


It's just loose on the piston. In the bore it's about 0.3mm. I think that's 12 thou, but my head doesn't work very well with imperial measurements.
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby dickie » Sun Sep 06, 2020 5:42 pm

Fast n Furious wrote:Rich,
Cant really tell from your posted photo but that ring gap line up your cylinder looks wey bigger than the expected 10 thou width I would expect to see.
Can you confirm?

Sorry Peter, I see what you mean now. That mark line up the cylinder wall is from where I seized it last year. It's a really shallow but wide 'score' mark which has subsequently filled with carbon. Ideally I'd have had a rebore and new oversize piston, but it's not that bad. I tried shining light up to see if it came through, but seemed good. Not sure how useful that test is, but it passed anyway :lol:

Edit: the piston is new, but the barrel has only been honed rather than rebored.
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby Warkton Tornado No.1 » Mon Sep 07, 2020 12:18 pm

If you are careful, one of the Sykes Pickavant type three legged honing tools could tidy that score. I wouldn't ignore it. In fact, I'd go as far as re-pegging if the score remained because having a gap in line with another is causing blow-by IMHO.

Honing tools are fine if they are as long as possible, though 4" (100 mm to you Dickie!) is as long as you may find @ a reasonable price. Driven really slowly with fluids such as paraffin or cutting oil. The trick to obtain the cross hatch is to go slow whilst traversing the bore. You may do it on a pillar drill but if you do not have access to one, I find that the Milwaukee 18v Combi drill has incredible torque @ very low RPM with no tendency to stall. Various grit stones are available.

Despite advice, I have run a honing tool down untidy Nikasil bores (you know the type....sh1te or bust) to gain a better evaluation. The plating is extremely hard, but as the result, the surface has some shine removed so that you can make a better assessment. Not everybody can afford re-plating costs if they can gain a few more miles from a bore..... :)
Warkton Tornado No.1
 
Posts: 2112
Joined: Sun Jan 11, 2015 1:27 pm

Re: Small block home tune

Postby dickie » Mon Sep 07, 2020 6:54 pm

Interesting idea mark. I was just going to ride as is until I'd finished my other project, then get 0.5mm oversize or something with a rebore.

Do you think it will increase ring gap enough to notice?
dickie
 
Posts: 1746
Joined: Thu Oct 02, 2014 12:32 pm
Location: Tyne and Wear

Re: Small block home tune

Postby Warkton Tornado No.1 » Tue Sep 08, 2020 12:10 pm

dickie wrote:Interesting idea mark. I was just going to ride as is until I'd finished my other project, then get 0.5mm oversize or something with a rebore.

Do you think it will increase ring gap enough to notice?


It's difficult to be definitive about a score like that, but if it were half a thou" (0.0005" [0.013 mm]) the resultant increase in bore circumference (hence increase in ring gap) would be just over three thou' 0.00314" [0.08 mm] so I guess you'd need to consider whether you'd want to stretch to an extra thou' in bore clearance. You do the maths! :lol:

Without re-reading, I can't remember what the clearance was :roll:

I'd happily send/lend you my honing kit (not the Milwaukee!) but I cannot, say, let you send me the barrel for fear of then 'interfering' further!

In any case, it's often the case that oversize rings can be regapped to suit, especially with Jap conversion pistons to optimise the gap, not that you probably need it in this scenario.
Warkton Tornado No.1
 
Posts: 2112
Joined: Sun Jan 11, 2015 1:27 pm

Previous

Return to Tuning & Kits

Who is online

Users browsing this forum: No registered users and 8 guests