low vacuum ( no leaks )

[Nige]

Hi,

I've just installed the MJ V4 on my 1963 Triumph Spitfire and all seems to be working fine, but I can't get the Kpa below 50-53.

The car is running it's 1147cc engine that has had a lot of balance and porting work done and has a piper 270 cam fitted, it is also running on keihin CR special bike carbs. I tapped into the inlet manifold and fitted the vacuum pipes. They've been off twice to check that there are no leaks and there aren't any (not in the manifold anyway). These carbs don't have butterfly valves, so it's just the slides blocking access to the outside.

If I just fit the manifold to the carbs and test the seal, it is letting a fair amount of air through the carb mouths and even when the slides are adjusted as far down as they can go, they aren't holding a vacuum. Is this normal with bike carbs of this type due to the fact that they don't have a sealing butterfly valve? If it is, can I get away with running the kpa as say, 30-100 rather than 0-100?, or will this cause bad things to happen?

TIA,

Nige

[NITROPIXIE]

I'm guessing your running twin bike carbs. What sort of setup do you have for the vacuum lines, sizes, lengths. Ideally you want real thin lines with a y-piece joining the two ports as far away as possible/reasonable. This may get you a few more KPA's and a more stable reading.

How fast is your idle currently?? If you have quite a high idle then you will naturally have a reduced amount of KPA at idle.

And have you got the fueling sorted out for the engine, at idle at least?? If you have a reasonable idle with low KPA then you could have quite an incorrect mixture possibly. A cost effective way of tuning at idle is to either get a colortune kit or ask your local friendly MOT station if they wouldn't mind letting you use there emissions tester for a small fee.

If you can get some photo's up of your setup then it's a little bit easier for us to see and understand how you have put it all together and give you some pointers. Especailly around the manifold area for tapping the manifold and vacuum lines, what manifolds you have used for the carbs, vacuum y-piece setup.

[Nige]

Yes, they are bike carbs from PRIRace.

You replied to me on my first post, pre-installation. There are pre-installation images of the carbs there.

http://www.autosportlabs.org/viewtopic. ... fire#12850

I tapped the rubber for the vacuum, I've used 4mm (internal measure) brass tubing for the tap and have two (2.5 ft) 5mm lengths of silicone tubing going to a 1.5x4" reservoir (made from a short length of sink overflow pipe with the ends sealed). All three taps in the reservoir have a 0.9 mm bore. There is then a short (1 ft) length of 5mm silicone tubing to the MJLJ.

I'll post pictures of the current set up when I get home from work tomorrow.

It is idling rough at around 750 rpm using a slightly tweaked default MJLJ map as I didn't want to get into setting up a custom map until I could get the Kpa's down. I didn't realise that the mixture could effect the vacuum, but I did adjust the slides right down and mess with the carbs concentrating on trying to bring the Kpa down. I didn't do a full re set up due to concentrating on the Kpa. I have a small home gas analyser and a colour tune, so I'll get on with getting the carbs fully back in tune and properly re-balanced tomorrow.

Thanks,

Nige

[NITROPIXIE]

Thats right i remember now. Nice little project.

You should be able to use a basic mini map for that setup to begin with as the engines aren't too different. You would at least then be very close to what you need anyway and there are plenty of them on here to choose from. Aim for 850 rpm at idle.

Did you get the timings marks on the engine to represent what you see in advance on the mjlj screen??

Have you tried squirting some wd40 around your manifold area. If the revs pick up then obviously you have a leak where ever you squirted.

Mixture can't effect the vacuum, but if you had the idle screw letting to much air in and you have adjusted the mixture to get the idle you want to see, could be a possibility. I understand you know how to tune the carb you have. I have always used single su's which are a bit simplier than this setup.

[Nige]

Hi,

It's been a while. I've been getting this running as best I can, but finally got around to buying a draper vacuum gauge. I wish I'd done this much sooner. The vacuum gauge shows that I'm getting 19 inHG at idle which is perfectly within its in its good range. After buying the gauge, a neigbor said he had one, so I have now confirmed this with two gauges.

The megajolt however is still showing 54 kpa. This seems to disagree with the inHG gauge, or is my working out wrong here?

None of this really matters much as the car is running fine, but what might cause the Megajolt to be reading the vacuum differently?

The car (1963 Triumph Spitfire) has a factory max advance of 39deg+-4 (zero vacuum) to 59deg+-2 (highest vacuum). The Megajolt has a maximum advance of 59deg. If the engine modifications call for a higher advance, could the global controller trigger offset be set to a plus value of say +3 and give a real advance 62deg, or is the cut off hard set at 59deg without tricking it by moving the sensor a couple of degrees?

The other question, and currently most confusing to me is about idle advance. As the carbs on this car originally used a ported vacuum on the carbs the vacuum at idle was full, yet the ignition timing was 15deg. As the throttle was opened, the vacuum was registered with the distributor and the timing was handled from there. Now that the vacuum is taken from the manifold, I have tried to simulate this by setting the idle rpm range to 15deg across the Kpa range, i.e all of 800rpm Kpa range is 15deg. Should I be doing this, or should I really allow the megajolt to read the vacuum and change the ignition timing. i.e 35deg advance at idle. There is no pinking at this advance, but I'm not sure whether I should keep the ported vacuum simulation set up, or just leave the megajolt to do its thing. See the attached screenshots to see what I mean.

To explain the kpa and rpm range values here. the kpa values are taken from the original factory service manuals and the advance is all in at 3000rpm which is why the range stops at 3500rpm.

Thanks for any input you might have on this, especially the idle advance values.

I've posted the factory specs along with the initial maps created from them and my current map file in the map library. http://www.autosportlabs.org/viewtopic.php?t=2631

Nige

[NITROPIXIE]

Hey, nice to keep us updated.

I suppose in the grand scheme of things it doesn't matter so much the amount of vacuum you do have as you can always adjust the vacuum axis to suit the engine, and so isn't a problem with the megajolt. Just keep the setting relative to your engine by keeping the same tables but changing the load axis to suit yours. 54 KPA is about 16 inmg so is within the ball park figure.

I'm not too sure about changing the values of the trigger off set to gain more advance, but you could physically offset the trigger wheel to gain extra advance if you needed to and alter the advance table to suit the correct advance required. I personally really don't think you should need this much advance. My A series has a maximum of 36-38 degrees advance at the top end.

My understanding is that the vacuum advance on cars is to improve the fuel efficency of an engine at the lower end of the rev range. All modern engines have a manifold vacuum port to which they recieve information of the engines load in order to give the correct amount of advance. Advance increases with rpm and an increase of load (less vacuum).

Also the reason for advance is that an fuel air mixture requires a set amount of time to burn before it creates it's maximum kinetic energy. The more air fuel mixture within the engine (acceleration) the longer it takes to get to this max kinetic energy point so more advance is required to get the maximum performance. So when idling not much air fuel mixture is used so a small amount of advance is required, when accelerating hard alot of air fuel mixture is entering the cylinders so alot more advance is required.

There will be no pinking with 35 degrees advance at idle as there is little/no load, but then at this advance your engine will be running hotter plus it's not good for the piston rings and you run the risk of them being fried with usage like this. Basically what happens is that the air fuel mix has reached its maximum kinetic energy while the piston is still coming up and i'm sure as you can appreciate this is not a good situation.

I believe what you have done here is incorrect and a continuous gradient is what is required, but an exact amount of advance is not required especially in low load areas like these, so it shouldn't make too much difference anyway.

Hope this helps

Ryan

[Desp]

My understanding is that the vacuum advance on cars is to improve the fuel efficency of an engine at the lower end of the rev range. All modern engines have a manifold vacuum port to which they recieve information of the engines load in order to give the correct amount of advance. Advance increases with rpm and an increase of load (less vacuum).

Also the reason for advance is that an fuel air mixture requires a set amount of time to burn before it creates it's maximum kinetic energy. The more air fuel mixture within the engine (acceleration) the longer it takes to get to this max kinetic energy point so more advance is required to get the maximum performance. So when idling not much air fuel mixture is used so a small amount of advance is required, when accelerating hard alot of air fuel mixture is entering the cylinders so alot more advance is required.


Ryan

Can you explain the above to me? I'm a bit confused on the subject and I am either hearing it both ways or am confused with my understanding of it. I was under the assumption that more advanced was required at higher RPM since the mixture always burns at the same speed it has to start the combustion sooner in order to reach maximum energy as close to TDC as possible. I understand that. What I don't understand is how load effects it.

Say you're under no acceleration and your driving down the road at 2000 RPM with 20 deg advance, so now you floor it, at that same RPM range do you want more advance or less? I'm being told if you increase the advance it'll ping, and you actually want to decrease advance under load as more mixture entering the engine will reach maximum energy sooner? Or is that backwards?

[DannyP]

LESS. I think Nitro had a mistype, as he corrected it later in that same post. More load=less advance, trust me. Aircooled engines are much more finicky and easily damaged by too much advance. Air doesn't as easily cool the hotspots of detonation like watercooled does. But at least you can hear it, sounds like ball bearings or marbles being shaken in a tin can.