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  1. #1
    Join Date
    Jan 2015
    Location
    Paignton Devon
    Posts
    296

    Rv8 3.9 problems

    Took the car for a longish trip at the weekend down to lands end, about 250 miles the first long trip on the new engine, anyway at cruise and acceleration it runs lovely the trouble is on very light throttle in traffic or around town it jerks and stumbles which is very annoying and can't be good for it, According to the wideband it idles at 13.6-1, at cruise it's 13.8 to 15.6, and it drops to 12.1 when you floor it, at very light throttle it is showing slightly rich about 13.1, which sort of suggests to me that I'm not far off for fueling, any suggestions for a cause?, could it be more distributor/ vacuum related?, I've just ordered a vacuum gauge so will be able to check the readings at the weekend , basic spec as follows,
    Rv8 3.9 ' interim' engine, fully stripped and rebuilt with all new bearings, rings and gaskets, new timing gear and rockers
    Real steel cyclone cam rest of engine standard
    Standard Lucas distributor with remote amp
    Timing is 14' at idle with the vacuum disconnected
    New engine and gearbox mounts
    Holley 390 carb on offenhauser 360 manifold, 56 primary jets, 57 secondarys, 6.5 power valve, steel colour vacuum secondary spring, 31 pump jet
    Bespoke s/s manifold and exhaust with new gaskets

    Any ideas or advice gratefully received as it almost spoilt what was a great trip
    Roger

  2. #2
    Join Date
    Dec 2008
    Location
    Exeter
    Posts
    127
    I remember some years back that Sidecarbod found that extending the accelerator pump operating arm, reducing the fuel squirt, helped with this type of problem on the Weber carbs. Does the Holly have a similar arrangement? If so a similar modification may help.

    John

  3. #3
    Join Date
    Jan 2015
    Location
    Paignton Devon
    Posts
    296
    On the Holley the fuel squirt is basically adjusted by different barrel jets, ( assuming the pump arm is adjusted correctly ) , Holley recommends going up in size for a lightweight car so that's what I did from a 28 to a 31, I could try going down in size to a 25 , I'm working away this week so will have to wait until the weekend to have a play

  4. #4
    Join Date
    Mar 2006
    Location
    Oxfordshire
    Age
    67
    Posts
    1,589
    Roger,

    The AFR readings look pretty close to me. There are a couple of things I'd check:

    1) Is the vacuum advance pipe connected to the "timed" (above the throttle plates) or "full-time" (below the throttle plates) position? In the first instance, it's worth swapping it over to see if it makes any difference.

    2) The usual accelerator pump tuning process is to change the accelerator pump cam for a different colour one (they have different cam profiles) and/or reposition it using the screw hole marked 2 rather than 1 (or vice versa). If you don't have a cam kit, it's probably worth switching the screw holes, again just to see if it helps.

    The vacuum guage will help a lot with the diagnosis! I suspect that you may see it fluctuating heavily under light throttle in traffic - in which case swapping ports may make a difference.

    The other possibility is the power valve. If you have very high vacuum at idle (say over 18 in Hg), you might want to go up to an 8.5. The idle vacuum on my 383 SBC is 20, so I run a 9.5 PV. The PV figure is the inlet manifold vacuum below which the PV starts to open. So a 6.5 PV will take need a larger throttle opening than an 8.5 to cut in - hence the need for a larger one if your idle vacuum is high.

    Hope this helps.
    Last edited by conrod; 09-07-19 at 02:05 PM.
    Alan

    CRC Thames Valley Regional Rep
    UK Rep for European Cobra Club (eurocobra.boards.net)
    Dax 427 DeDion - Huddart SBC 383

  5. #5
    Join Date
    Nov 2008
    Location
    S/W Cornwall
    Posts
    425
    Try running it with the vac system disconnected and blanked (so the manifold is not sucking air) See how that is.

    If it doesn't like that check your all in advance should be around 36 at 2700-3000 RPM with no vac system used. (if its not adjust your timing and try that)

    Simon

  6. #6
    Join Date
    Jan 2015
    Location
    Paignton Devon
    Posts
    296
    Thanks for the suggestions, at the moment the vacuum is connected to the timed outlet, I must admit I hadn't thought of the cam profile, all things to try at the weekend when I can play with the new gauge and I'll hook up he timing light again as I can't remember my all in figures

  7. #7
    Join Date
    Oct 2006
    Location
    Toton, Nottinghamshire
    Age
    46
    Posts
    837
    Another Sidecarbod gem I remember was that Lucas made a bit of a foul up with their vacuum advance can design, namely that it applied way too much advance. Rover's "resolution" was to reduce the static/idle advance setting to compensate. I think the standard setting for use with 4-star fuel was 10deg BTDC and somewhere around 6deg for unleaded. Try reducing your timing or disconnecting and plugging the vacuum hose and see if that improves things.

    Russ.
    Joint Area Rep - East Midlands Region
    1984 DAX Cobra
    Rover 3.5 V8 with Holley carburetor.

  8. #8
    Join Date
    Jan 2015
    Location
    Paignton Devon
    Posts
    296
    An update on progress, I hooked up the vacuum gauge and the vacuum at idle was low but steady, I've increased the timing until I'm all in at 38' at 2800rpm this gives me 16 hg of vacuum at idle but 18' of timing at idle, seems high but... anyway went for a drive around with the vac disconnected, it goes well the partial throttle in traffic stumble has gone, I still think there is more to be gained from the engine though, if I go solely by the vacuum gauge I could advance the timing even more to get a higher idle vac but I'm not sure I should as that will put me up around 22' advance at idle with the vac disconnected , its not pinking or anything at the moment and drives pretty smoothly so it may take a bit more, what do you reckon?
    Roger

  9. #9
    Join Date
    Mar 2006
    Location
    Oxfordshire
    Age
    67
    Posts
    1,589
    Hi Roger.

    You could try advancing the static timing further, but at the first sign of pinking under load back it off again. The other thing to be aware of with a lot of idle advance is that it can make the engine more difficult to start, with spitbacks through the carb. But if you're not experiencing either of these, and your AFRs are OK, then you should be good to go. I don't know whether you've only hooked up the AFR and vacuum gauges temporarily - if so, it's well worth making them permanent additions to your dashboard, as between the two of them you have an early warning system of many engine problems, and two great diagnostic aids to solving them!
    Alan

    CRC Thames Valley Regional Rep
    UK Rep for European Cobra Club (eurocobra.boards.net)
    Dax 427 DeDion - Huddart SBC 383

  10. #10
    Join Date
    Jan 2015
    Location
    Paignton Devon
    Posts
    296
    No problems starting, in fact 1st start of the day it idled easier than it did until warm and hot start is easier although it wasn't really a problem anyway, I've been playing a bit more this afternoon and as the idle afr was showing rich at 12:1 I leaned it off a bit, that seemed to bring back the stumble but nowhere as bad as it was, so I've now richened it up again, I think tomorrow I'll advance a bit more and see what happens I'm slightly concerned that the big advance setting is masking another problem although the vac and wideband readings seem ok, the gauges are only temporarily hooked up at the moment I'm not sure where I would put them and still look neat, perhaps I could devise a hinged drop down plate to house them, that's for the future though once the engines running how I want it I've got a hardtop to finish and paint ready for the winter

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