10 replies to this topic

[Gsrmini]

Recently purchased, I'm pretty sure 998 fitted.
Trying to sort out a number of issues, as I plan to be daily driving it.
I havn't determined size but its the single su carb with external float bowl. Cone style filter fitted directly to carb back.
To start Ive been experiencing the same 'heavily discussed topic' of hesitation/coughing/dying under throttle, especially load. (completely non-drivable) slightly hard to start-up, but seems to idle very nicely.
My dashpot oil was low & piston possibly a bit dirty. I've topped up w/ 20w-50, and wd40 cleaned the piston & housing.
This obviously helped alot, starts much better, I am able to drive it a bit now. Though slightly drivable, still experience the same symptoms w/ giving throttle... Especially when under load.
The previous owner removed alot of the emissions related system, including vacuum to the carb. Could this be contributing the issue? Do many people run w/ the carb vacuum fittings capped?
Just trying to figure out what next? As I've topped up the dashpot oil, which helped lots, but not solved. I figure I'll probably take the top off the float chamber see if I happen to visually see an issue.

Currently:
Dashpot oil full (20w-50)
Carb piston/housing cleaned up a bit.
Plugs look slightly white/grey tips not black, or wet.
Rotor cap slightly darkened contacts, but I don't think enough to be "the" issue. Will be replaced at some point soon though.

Thanks in advance for suggestions. This forum has been a huge wealth of excellent information, and I've only ever needed to read to find the answers, this'll be my first actual post for suggestions.

Thanks,

[carbon]

When you say 'vacuum to carb' do you mean the engine breather pipe which comes of the engine block? This may be going to the tappet cover at the back of the engine on a 998, or maybe onto flywheel housing. On the HS4 carb which it sounds like you have fitted on your motor there is a brass tube at the side ofthe main carb body level with the dashpot piston, and the breather pipe is often connected into here as std.
If the pipe is removed without balnking off the pipe the carb will tend to run lean, as air gets in and dilutes the mixture. With the breather connected the amount of air allowed in is much reduced.
Your symptoms sound like running lean, could be related to fitting a less restrictive 'cone' air filter.
Do you know what needle you have fitted? If this has not been replaced recently would suggest getting a new std spec needle as first step, and also at same time try fitting a new dashpot spring - if Minispares still have any of the recent batch of 'red' springs for the HS4 they are a little heavier and may help to reset the mixture. With this spring you may need to 'lean off' the adjustment at idle to get the mixture correct for tick-over, but will help richen it up slightly under load.

[bmcecosse]

You need the vacuum connection from carb to dizzy - and the 'fume suction' system between carb and rocker cover/cam follower chest etc. Any connection lying open will result in horrible gasping and weak running.

[Gsrmini]

Thanks for the suggestions. I too understand it to be running lean, and somehow being fed too much air under throttle/load. There is no tube to the carb, however it has been capped off. So in theory as long as properly capped it shouldn't have much negative effect to be setup this way?

Previous owner has removed alot of emissions/breathers/vacuum etc lines. Also has the hose from the Oil separator/flame trap plugged off with a bolt. Which normally runs to the carb, correct? (I'd hesitate to reconnect at the moment due to the amount of buildup in there though) its in such an awkward location too, that I'm not sure how to remove it for cleaning.

I'll pull out the needle and see which is fitted. also not sure of the current spring.
Unfortunately if requiring parts, it becomes a bigger issue for me to source and ship.

Thanks,

[liam1288]

just one on that, i had mine tuned on the RR at bogg brothers and he removed my dizzy to carb vac pipe... said i didnt need it and blocked off the connection where it goes onto the carb.
i have thought about connecting it back on but didnt want to mess up the tuning he did costing 80 quid

[bmcecosse]

The vacuum connection between carb and dizzy is ESSENTIAL for a road going car...... What a waste of £80.......

[Gsrmini]

You need the vacuum connection from carb to dizzy - and the 'fume suction' system between carb and rocker cover/cam follower chest etc. Any connection lying open will result in horrible gasping and weak running.

Carb connections are capped at the moment. Also I imagine your referring the same 'oil separator/flame trap' vent as I just had?
We were posting at the same time, see above. Mines plugged w/ a bolt, full of grime, not sure how to remove to clean before reconnection.
Understandably would rich'en the mix a bit if reconnected. Should I try to get that all hooked up again?

[liam1288]

hmmm he does know what hes doing and tunes a lot of racing cars...maybe he just expects me to drive at full throttle everywhere! haha if i put it back on, it will advance too much at faster speeds wont it? he said the timing was right without it on... i have a feeling it read 36 on the strobe light but not sure...
sorry for hijacking the thread in a way too :)

[Gsrmini]

The vacuum between dizzy and carb, as mentioned. Looking at the car now I have no idea where from in the general area of the distributor that vacuum line would connect, and to which port on the carb?
I see the port at carb piston height sticking out at a 45 degree angle, that would be for the oil separator/flame resistor vent hose, correct?
But I don't see where the dizzy/carb hose would run.

Thanks,

[Pigeonto]

hmmm he does know what hes doing and tunes a lot of racing cars...maybe he just expects me to drive at full throttle everywhere! haha if i put it back on, it will advance too much at faster speeds wont it? he said the timing was right without it on... i have a feeling it read 36 on the strobe light but not sure...
sorry for hijacking the thread in a way too :)

...No it wont.As BMC says,most road going cars need the vacc advance because fuel economy is normally an issue.You can use more advance on cruising-part throttle,and idle-no throttle (vaccuum present in the inlet manifold) which will give better ecomony.So the vacc advance mechanism will pull and advance it up.You cant have so much advance on full throttle or any accelerating,climbing hills etc and that is when there is pressure,not vaccuum in the inlet manifold and pinking is likely. In this situation without vaccuum present the vaccuum advance isnt functioning so the timing retards to prevent the chance of pinking.Race cars dont cruise,they are either accelerating or decelarating,no inbetween so they can have a total advance set and locked just below pinking point.

[Gsrmini]

So it's been determined that the vacuum between distributor & carb is beneficial to daily driven cars.
Can anyone tell me where that line is properly plumbed to & from?

However before I worry about fuel economy from vacuum dizzy-carb, I still need to sorted out why my car is 'leaning' itself out under load & throttle. Couldn't check my carb needle yet, sorry.

Anybody with thoughts?
Or how to remove the oil seperator/flame trap for cleaning?