Good evening. Mr. Modivator here. Murf has been getting hard to start from cold and showing the exact symptoms that the Haynes manual says points to "blocked starter jets, passages or valve".
So the carb is coming off anyway for a good look at those areas.
Meanwhile I have a 28IMB1 fitted to the 594 engine, which I have had working. So partly as an exercise to double-check and eliminate other reasons for the issue and partly for fun, I'm going to try the bigger carb.
I've checked the specs and apart from the main jet, which is slightly bigger, there isn't much difference from the 26. There is a need to swap one of the carburettor mounting studs, which is longer and the intake elbow will need the later type intake tube, but apart from that it's a straight swap.
To the question, has anyone done this and WILL I GO EVEN FASTER?

It is probably worth a try but the inlet port / hole in the top of the head is 26mm on the 500 And 30mm on a 126. Yes the stud pattern is the same and the base flange on the 28IMB is stronger therefore less prone to warping. I have a ported head on my 500 engine , a 30mm inlet and a 28IMB carb.

I put it on and it starts and runs. Initial thoughts are that it's a bit smoother in acceleraton but as always ths is subjective and probably just me being influenced by he knowledge something is different.
The engine was no easier to start from cold but as the carb would have needed priming maybe I will be lucky. But I'm thinking my starting issue is elsewhere.

Mr Modivator I can see it's the continued slide down the 'Modding' banister here Bigger Carb = more Go in the snow:worship::worship::worship:
""So partly as an exercise to double-check and eliminate other reasons for the issue and partly for fun"" Oh yea - sounds like a good excuse to 'Mod' Now I must finish my carb cleaning just to eliminate other reasons

Ian.

Bleeding Knuckles said:

Mr Modivator I can see it's the continued slide down the 'Modding' banister here Bigger Carb = more Go in the snow:worship::worship::worship:
""So partly as an exercise to double-check and eliminate other reasons for the issue and partly for fun"" Oh yea - sounds like a good excuse to 'Mod' Now I must finish my carb cleaning just to eliminate other reasons

Ian.

Click to expand...

...you read my mind Ian.
However, this particular and very first modification I have made (all others being mere accessorising) has proved to be abortive.
After running OK for a while, the engine decided it wasn't gong to start AT ALL.
I don't blame the carb..there is something else going on.
So we are back on the original carb and mobile again. But the engine wouldn't start until I had replaced the recent, proper Fiat, new plugs using my spares, which are Champion and have already done 6000 miles. Even then I had to torch the plugs to an unbearable temperature before fitting in order to overcome the petrol-flooded conditions in the bores.
These engines can be temperamental little things.

Ultimately the engine wouldn't start with the 28; it was only an experiment so I re-assembled everything with the 26IMB.
I had done a partial autopsy on the 28 to see the differences from the 26 and today I spotted the problem whilst reassembling it. It had the infamous petrol in the float problem so I guess that is why is didn't work.
The chances are that this was was one the mystery things that might have helped drive someone mad enough to abandon the engine it was fitted to and which I then bought for a bargain price.

So all in all is it worth put a 28 on or better go for a 30 if the 500 can take it of course. I think that my 26 has issues even if I overhaul it so thinking to go bigger and faaaaster

Thomas

gordinir8 said:

So all in all is it worth put a 28 on or better go for a 30 if the 500 can take it of course. I think that my 26 has issues even if I overhaul it so thinking to go bigger and faaaaster

Thomas

Click to expand...

It seems that the engine will work OK with the 28IMB, but it is not a performance booster!
I think I am happy with my 499cc for a few months longer and then I will fit the 652cc when I want more power.
I also have a nice little 594cc engine which I would like to play with. Maybe that is for next winter.