rejetting and rebuilding

[pyro]

hi all, i have a weber 34/36 dmtl carb which i will be rebuilding at some point, just wondering how i would go about it and what is involved in re jetting a carb?
at the moment its jetted for a land rover 2.5 petrol, but im considering putting it on the 360 as i believe the mounting is the same and its bigger than the solex?

oh yeah tappets on a b200k how do i go about getting them done as the heinz book of lies isnt much use! it sounds like a diesel on a cold morning.

thanks - ben

[pettaw]

Rejetting a carby, not easy on the road. To do it properly you need some sort of sensor to attach to the exhaust to monitor the fuel mixtures and then a dyno to put different loads on the engine.

You can do it by the 'butt dyno' and try and work out where its lean and where its rich if you've got lots of open space to play about with it, and then experiment where you need to change jets/tubes.

Working out where its rich and where its lean is half the battle. If you're rich, the car runs OK, but it seems to lack power, it feels boggy and hesitant and generally not happy, but otherwise sounds OK. Lean the car sounds crappy, it may start misfiring slightly, or sputtering. Slightly too rich or too lean and it can get confusing and you can start getting lead in the wrong direction. I've certainly got confused when I've been trying to cure hesitations on my EFI setup and started adjusting things without looking at my fuelling readouts.

This is the basic theory of carb jetting, I've only ever really played about with things that are supposed to have been correctly jetted and been put back together wrongly etc, but anyway....Generally speaking, idle jets control the low down stuff. Idling and off-idle throttle, maybe the first 10-20% of throttle, before the main jets start working properly. The main fuel jets and main air jets control the full bore stuff, the fuel controls all of it from off-idle to full bore, and the air modifies the very top end stuff, with the emulsion tubes controlling how the two of them add fuel and air progressively. Trouble is that one section doesn't control one part, they all interact and overlap so you change one thing you've then got to check how it affects everything else.

Then you've got your acceleration enrichment. When you stomp on the throttle, you switch on a whole load of air, before the calibrated jets have had a chance to add the correct amount of fuel, so you get a lean spot and the engine will hesitate. To lessen this, most carbs have an accelerator pump, which squirts a calibrated amount of fuel down the main venturi of the carbs to increase the fuelling for a second before the main jets have caught up. As usual though, this amount needed is also affected by the emulsion tubes and the main jets, because the extra amount of fuel also comes from the amount of fuel stored in the emulsion tubes.

The tappets is easy
You'll need a screwdriver, the Ovlov special tool for removing the tappets, feeler gauges to measure the gaps and a micrometer or calipers to measure the tappet shims. Its best to do it with the engine cold. You don't need to follow all that procedure in the Haynes where valve x is open, valve y is shut etc etc. Its obvious...when the cam lobe points upwards, the valve is closed and you can measure the gap. The easiest way is to turn each cylinder in turn onto TDC compression stroke ie: where the inlet and exhaust are both closed and pointing equal amounts away from vertical, then you can measure the clearances with your feeler gauges. Find the thickest feeler gauge you can fit into the gap. If any are out, then write down how much they're out by.

Then once you've got all your measurements you can use your special tool. Turn the engine over a few degrees so the top of the piston is below TDC otherwise you might hit the crown when you depress your tappet down. Then you wind your special tool down, and use a screwdriver to prise out the tappet shim from the tappet bucket. Once its out, you measure the shim with your micrometer or calipers to work out its thickness and then see what new size you need. They come in increments of 0.05mm so pick the closest size. Do that for all the ones you found that were out and then pop off to the local stealer and order in new shims.....fitting them you rub lightly in engine oil, and put face down into the tappet bucket, then release the special tool gently, and the shim will seat home.

Hope that helps.

[pyro]

Ok thanks for that!

but excuse my ignorance. special tool? are they hard to come by or will something else suffice? failing that is a garage better to do it?

thanks - ben

[volvosneverdie]

Ok thanks for that!

but excuse my ignorance. special tool? are they hard to come by or will something else suffice? failing that is a garage better to do it?

thanks - ben

.You just said, Tool, Come, and Hard in the same sentence.
Immaturity Ruuulez.

[pyro]

i cannot believe i missed that!

Mr Cadbury met Miss Rowntree in a room on Quality Street. It was after 8. He turned out the light for a bit of Black Magic, then slipped his hand into her Snickers and felt her Curlywurly. Not keen to have any Jelly Babies she let him have her up the Chocolate Truffle. She screamed with Turkish Delight as he took out his Fun Sized Mars Bar. It felt a bit Crunchie and she wanted some Time Out but he did a Twirl and came in a very Milky Way!!

and oh yes it does.

Ben

[volvosneverdie]

Mint! (aeros).

[pyro]

Sweet

[tommysb]

I know a little 'story' like that all based around maths....How sad is that?

[volvosneverdie]

I know a little 'story' like that all based around maths....How sad is that?

76.5 percent sad.

Given that 100% is the accepted maximum.

[pyro]

anyway - tappets lol!

might start an 'immaturity corner' thread later on, think it would be appropriate!

Ben

[MJ]

Or inappropriate, as current proceedings would suggest

[pyro]

Or inappropriate, as current proceedings would suggest

you whaty wot wot?

Ben