GT186 manifold - carb fouling engine mount

[vegansydney]

I'm the final stages of an GT186 build and have discovered that the carb, a Mikuni TM24, won't fit as it fouls the engine mount. I found it strange because the GT Assembly Manual specifically mentions the TM24 and suggests 'ballpark' jetting. Searching the archives here also suggests that a number of others are also using this combo.

To complicate the issue further, there appears to be two versions of the 25mm Gran Turismo manifold. Mine is the later version that is upswept.

Since discovering the issue, I've also spoken an experienced builder and they confirmed that in the past they've cut the 25mm manifold and re-welded it at a more accommodating angle so the TM24 clears the mount. I'm willing to go down this route (and change it to a flange type mount while at it), but seeking advice from others running a TM24 on at GT186 first. Thanks!

[swissdean]

I used offset cone in mine and it fitted, this is likely the way you need to go.. mine was the upswept one also

[vegansydney]

Great to know, thank you. Any chance you could post a picture of the clearance, or DM me a photo. It would be much appreciated, thank you!

[John Deeley]

Offset cones will only give more clearance between the manifold and frame tube, they will not change the relationship between carb and engine mount as both are in a fixed position on the engine block.

[vegansydney]

A quick update:

In order to fit a Mikuni TM24 I cut the 'upswept' end off a 30mm GT manifold and rotated it approximately 90 degrees so it pointed forward. I then cut the end off flush. After measuring clearance more times than I care to count, a laser cut flange, that I specifically designed to fit a BGM flange manifold rubber was welded on.

Why? I wanted to run a Mikuni TM24 with the option of using an airbox. I figure if I was going to modify it, should go to the effort of converting it to the superior flange type mount. If anyone wants a DXF file of the flange mount to match the various BGM flange rubbers send me a message with your email address.

[Warkton Tornado No.1]

Out of interest, does the Mikuni carburate well with the set up? I would imagine that it does, though (IMO) it has the advantage of being not only two stroke specific (with a guillotine slide similarish to the OEM SH's) but additionally not liable to flooding even a nat's with the consequence of difficult hot starting.

Presumably your choice of Mikuni may be down to personal preferences, involving experience, brand loyalty etc, but did you also look into compactness by comparison with any viable alternatives?

Excuse me being nosey, but gathering knowledge such as this can be hugely beneficial. Besides which, I gain the impression that as far as two stroke motors are concerned in your part of the World, the likes of Keihin, Mikuni etc are probably the market leaders & therefore more of a natural choice.

TYIA

[vegansydney]

Here's another picture. As you can see it's still super tight, but there is now ~2mm of clearance between the engine mount and choke mechanism and slightly more (>2mm) between the carb body and engine mount. Overall, the manifold needs to be moved ~7mm forward for it to fit the Mikuni TM24.

Out of interest, does the Mikuni carburate well with the set up? Presumably your choice of Mikuni may be down to personal preferences, involving experience, brand loyalty etc, but did you also look into compactness by comparison with any viable alternatives?

I've been using Mikuni TM24s for around a decade as they're a small, Japanese-made flatside. I've found them to be super responsive and affordable. They can be purchased for as little as $115 (~£85). Jets are also wildly available. They use 30mm ID mounting rubbers so PHBL manifolds (GT manifolds being the odd exception!) can be utilized. The only downside I've found is slides and atomizers are slightly harder to find (not all Mikuni suppliers carry the entire range), but that said, I've had no trouble getting what I need by shopping around. Personally I'd prefer the fuel inlet to be on the RHS, but it's only a very minor issue.

I've regularly heard people complain they could never get TM24s set up right. This, I believe, is due to them using the stock atomizer (454 Series Q0). In my experience, atomizers need to be richer (454 Series Q3 or Q4), and slides leaner (4.0+), than what the TM24 come with from the factory. For piston-ported set ups with Clubman type exhausts, I've found (unsurprisingly) that pilot jets need to be smaller (10, 12.5, 15). All of these recommendations are at odds with most of the recommended jetting set ups available online, but I've verified through multiple jetting runs with a wideband UEGO sensor and complemented with plug chops.

I did toy with the idea of a Polini 24mm PWK clone when I found the TM24 wouldn't fit the GT 25mm manifold. I like that the PWKs are so compact, but the issues with float bowl setting, fuel flow and chrome flaking off slides on the PWK clones (Polini, OKO, Stage 6, Koso, unbranded etc..), along with the added investment in jets made me reconsider the TM24. For a larger displacement engine (or different tune) I would consider a genuine Keihin PWK 28. If there was a genuine 24mm made by Keihin I'd seriously consider it, but for now I'm not interested in PWK clones (licensed or not).

[Storkfoot]

In the States, TM24s are cheap. When I bought one last last year, I couldn’t find one in the UK. I ended up buying one from one of the German companies but it was about £140.

Post Brexit, with Customs tax on top, they are going to be even more expensive.

I am hoping that they become more popular in the UK and one of our dealers starts stocking them again.

Nice work with the manifold by the way

[Warkton Tornado No.1]

Out of interest, does the Mikuni carburate well with the set up? Presumably your choice of Mikuni may be down to personal preferences, involving experience, brand loyalty etc, but did you also look into compactness by comparison with any viable alternatives?

I've been using Mikuni TM24s for around a decade as they're a small, Japanese-made flatside. I've found them to be super responsive and affordable. They can be purchased for as little as $115 (~£85). Jets are also wildly available. They use 30mm ID mounting rubbers so PHBL manifolds (GT manifolds being the odd exception!) can be utilized. The only downside I've found is slides and atomizers are slightly harder to find (not all Mikuni suppliers carry the entire range), but that said, I've had no trouble getting what I need by shopping around. Personally I'd prefer the fuel inlet to be on the RHS, but it's only a very minor issue.

I've regularly heard people complain they could never get TM24s set up right. This, I believe, is due to them using the stock atomizer (454 Series Q0). In my experience, atomizers need to be richer (454 Series Q3 or Q4), and slides leaner (4.0+), than what the TM24 come with from the factory. For piston-ported set ups with Clubman type exhausts, I've found (unsurprisingly) that pilot jets need to be smaller (10, 12.5, 15). All of these recommendations are at odds with most of the recommended jetting set ups available online, but I've verified through multiple jetting runs with a wideband UEGO sensor and complemented with plug chops.

I did toy with the idea of a Polini 24mm PWK clone when I found the TM24 wouldn't fit the GT 25mm manifold. I like that the PWKs are so compact, but the issues with float bowl setting, fuel flow and chrome flaking off slides on the PWK clones (Polini, OKO, Stage 6, Koso, unbranded etc..), along with the added investment in jets made me reconsider the TM24. For a larger displacement engine (or different tune) I would consider a genuine Keihin PWK 28. If there was a genuine 24mm made by Keihin I'd seriously consider it, but for now I'm not interested in PWK clones (licensed or not).

My understanding is that Keihin license out the manufacture of sub 30 mm PWK carburettors & that there are therefore "genuine" PWK's that are manufactured by Koso, OKO, Polini etc. Such products should be trustworthy. After all, some are even 'OEM' on some engines.

The problems that are associated with the PWK's have come about due to poor counterfeits, but some of even those bootleggers have cottoned on to the fuel supply issues, just as the trusted manufacturers have long ago. Like any misdemeanour, "mud sticks" & consequently the manufacturers that do conform to the Keihin standards suffer too. Checking the real thing from fakes is not difficult & companies such as Mid Atlantic Trials that have invested in OKO go to some length to achieve that, to the extent of even being constructive in getting the bootlegs operable.

However, it pays to check.......as should be done with ANY carburettors.

The problem with counterfeit products is rife. I've given up with buying any more NGK 'plugs unless of a specific type unavailable. The common or garden road engine grades that I have just seem to fail, so I have reverted to Champion, Denso etc.

[Storkfoot]

Any progress on this, Vegansydney?

[vegansydney]

The engine is built but the frame is off getting 'jigged' before paint. The closest Lambretta-specific frame jig to New York City is 27 hours drive away, so might be a minute. Will post more photos when it's back. Itching to get this one done!

[Storkfoot]

I am now planning to run a TM24 on my GT186 with an early MB/ bgm Clubman. At present I am running a PHBH30 and have done for many years. I only get maybe around 12 miles to the litre and fancy better fuel consumption and a better throttle response.

Unlike Vegansydney, mine will clear the engine mount as I am running a 115mm conrod with 8mm packer as well as the Casa engine bolt system. I do not yet have the stepped MB rubber I have linked below but I can tell that the carb will easily clear on all sides. The one possible issue is that the stepped rubber is 30mm on the carb side whereas the Mikuni carb spigot is 31mm.

https://www.lambrettaspares.com/spares/ ... p0435.html

[vegansydney]

Nice work. Great to see it will fit!

I originally calculated that the manifold needs to be moved ~7mm forward for it to fit, but the looks of your picture it could be closer to 5 or 6mm. Please post some photos when you've finally fitted it. Thank you

For jetting piston-ported engines I've found that 454 Series Q3 or Q4 atomizers and slides of 4.0+ are needed. I've not jetted mine yet (still waiting on the frame to come back) so I'd be interested in how you get on. I've never had a problem using 30mm (PHBL) rubber manifolds on the TM24s.

[Storkfoot]

Nice work. Great to see it will fit!

I originally calculated that the manifold needs to be moved ~7mm forward for it to fit, but the looks of your picture it could be closer to 5 or 6mm. Please post some photos when you've finally fitted it. Thank you

For jetting piston-ported engines I've found that 454 Series Q3 or Q4 atomizers and slides of 4.0+ are needed. I've not jetted mine yet (still waiting on the frame to come back) so I'd be interested in how you get on. I've never had a problem using 30mm (PHBL) rubber manifolds on the TM24s.

Will do. I have found a Mikuni supplier very near to me. He has the carbs in stock but not the atomisers and slides you refer to. He has emailed Mikuni for me this morning asking if they can supply him with them.

When I last ran a TM24 on a piston ported 185 several years back, I didn’t need anything other than the standard internals, with one exception, a 4DH7 needle. However, it would be good to have those options at hand.

I won’t be fitting the carb until I come back from a tour of Scotland next month but I’ll keep you posted then, Benji.

[Storkfoot]

Well, the benefit of an 8mm packer is that the carb is a good fit. The downside is getting exhausts to fit

[Warkton Tornado No.1]

With many engine configurations, removal of the barrel complete with 'U' pipe can become impossible with engine in situ due to the splash plate.

Last time the issue reared it's ugly head, I made another splash plate from an old recycling bin lid (black plastic) & made it removeable. I hot-air-gunned it to bend forward, rather than back, made it a bit deeper & symmetrical about the centre line.

[Storkfoot]

With many engine configurations, removal of the barrel complete with 'U' pipe can become impossible with engine in situ due to the splash plate.

Last time the issue reared it's ugly head, I made another splash plate from an old recycling bin lid (black plastic) & made it removeable. I hot-air-gunned it to bend forward, rather than back, made it a bit deeper & symmetrical about the centre line.

From experience on the road, I can vouch for the fact that this issue does not present itself with this barrel

[ROClarke]

With many engine configurations, removal of the barrel complete with 'U' pipe can become impossible with engine in situ due to the splash plate.

Last time the issue reared it's ugly head, I made another splash plate from an old recycling bin lid (black plastic) & made it removeable. I hot-air-gunned it to bend forward, rather than back, made it a bit deeper & symmetrical about the centre line.

Did it give you more down force !

[vegansydney]

Niiice work! Are you going to run a filter? The fact that the TM24 (and TM28) can fit GP/dl 150/200 air hoses are just another reason why they are such a great little carb.

Wish they fitted with that much room without the 8mm packer. If you want to convert that manifold to a flange type, send me a message and I'll get some more flanges laser cut.

[Storkfoot]

Niiice work! Are you going to run a filter? The fact that the TM24 (and TM28) can fit GP/dl 150/200 air hoses are just another reason why they are such a great little carb.

Wish they fitted with that much room without the 8mm packer. If you want to convert that manifold to a flange type, send me a message and I'll get some more flanges laser cut.

Thanks. Yes I am going to run a filter. I have a couple to try.