For the sake of consuming bandwidth here is that 'other' email. I have some
drivability problems that I just can't shake.
In mid to med-high throttle openings I get a constant miss, similar in
feeling to running out of gas; light throttle is ok and WOT is also ok.
This problem is irrespective of mixture as I have tried dozens of settings
ranging 20% each way of stock, the only change I ever see here is that when
I go too lean, all drivability goes to hell.
Secondly, if I try to pull from 1-1.5k rpm up to speed the engine will miss
badly regardless of throttle position, this will never clear up until I
release the throttle and redepress it.
Lastly, my tach overreads above 3800 rpm, if I rev hard the tach will hit 7k
when the engine is turning 4k. If I drive slowly to speed the tach will
read smooth until I hit 3800 rpm at which time it will jump 1k rpm and
maintain that or larger overread.
The first two problems appear to be partially timing related and the last
seems to be related to point bounce. I used an adjustable timing light to
map out my advance curves on both distributors and here is what I found.
vacuum advance starts at 12in and is full in at 21in, it is connected to
ported vacuum as instructed. The first distributor, the 'recurved' one, I
have no history of but is stated to have been recurved. The second one is
the stocker from the 2bbl auto engine that I have in the car.
On the recurved dizzy the mechanical advance started to come in at 800 rpm
and had 40 degrees in by 2800 rpm. On the stock dizzy the advance started
to come in about 1k and had a total of 21 degrees in by 4k. it seems that
the recurved one had too much advance and the stock one too little compared
to the advance graphs given in the shop manual.
All of the above symptoms were constant with the recurved distributor. With
the stock one, they are fairly minor at first but once the engine has warmed
up they manifest themselves just as badly.
The engine runs at 180 deg but my engine bay is very sealed and I think that
may be part of the problem. The front of the engine bay is sealed and
ducted to the radiator, the headers exit directly above the front wheels and
are ceramic coated inside and out. What I believe to be the real killer is
that I am using a 1.5" drop base aircleaner with an airbox ducted out my
hood scoop, so I am not using any of the engine bay air. My guess is that,
because I am not circulating any air the carb is just baking in the stagnant
air and nearly if not totally boiling the fuel.
I know that others have had problems with fuel percolation and have
eventually solved them; my current idea for solving them is to move the
bottom of my airbox under the carb and insulate it so that the intake air is
flowing over the carb body. My concern is with how the airflow into the
carb will be affected by having an airbox encasing the whole carb rather
then one that funnels all the air towards the inlet.
So, I'd like some opinions as I need to fix this before I do cam stuff.
1. since we talk about a 6k rpm valve float the points ought to get up there
without bouncing, so what other then points could be causing my overreading
situation?
2. what are everyone's opinions of my timing curves and do either of them
sound problematic?
3. does anyone have an opinion of the effects on airflow to alter the airbox
as described, effectively leaving the carb to draw through the open top but
sealed in an environment of filtered air.
4. will I require the addition of an airhorn to help direct the flow of air
into the carb.
Once again, thanks for any insight on these issues and for reading this far.
I'm beginning to appreciate the things said about sorting a custom car.
James Nazarian
71 MGB Tourer
71 MGBGT V8
85 Dodge Ram
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