1st Gen Power Problem

[rx761]

I have an 81 RX7 with a street ported 12A that I run with NASA. I am having a problem and can not figure out what the cause is. When the RPM's get to 7000, it is like I am hitting a rev limiter, and the engine starts missing. My fuel pressure does not drop, and all my gauges appear to be fine. I have a weber 45dcoe carb on it, and it has been tuned on a dyno with no RPM probelms. I have changed spark plugs and plug wires and cleaned the fuel filter, and also have changed the distributor cap, but nothing has helped. I do not know what else to check or try. Anyone have any ideas?

[seckerich]

Are you running points or some other spark distribution? Points will get weak and bounce causing the problem you mention.

[lateapex911]

I don't recall any RX-7 using points...
As this seems to be a new problem, has anything happened or changed?
Have you tried swapping coils?
As always, check all elec connectors and connections and wires. The old harness can get pretty sketchy.
Has the carb* secondary broken/jammed/clogged? (perhaps one jet is a bit clogged? the old tanks get some very fine rust in them that can build up, even get past the fuel filter, not to mention other stuff)

*I'm not familiar with that particular Weber, so excuse me if my thoughts are way off on the carb aspect)

[NCRX7RACER]

The 1979 rx7 has a dual points setup.

[rx761]

No points, I have a MSD setup and coils are new. Carb was re-jetted when it was on the dyno, so jets should be good.

[lateapex911]

I assume compression is good, right? One thing to remember about MSDs is that they, in my experience, like a good clean current and solid wiring.
Also, is there any chance a "Pill" got swapped /changed that is the rev limit? (My MSD is older now, and had some 'pill' thing you used as a rev limiter. )
Or it's counting revs wrong and thinks its at redline....?

[dickita15]

If you have a stock distributer kicking around it would be worth the time to stick it in to eliminate the MSD as a possible cause.

[rx761]

The problem started before the MSD was installed, and the current motor. I just had a street ported motor put in, and the msd installed, but the probelm is still there. So the MSD should not be the issue. It did start however after the weber carb was put on, but I do not know why on the dyno it will go past 7000rpm, but not on track. Carb was new when installed, and jetting has been adjusted.

[Dano77]

Pretty sure you just answered the question your self. The prob was there BEFORE the SP engine and The MSD. Leaves Fuel delivery.

The reason it wont show on the dyno is that the car is not subject to Gravity,inertia,G loads and all the other stuff associated with a car that is MOVING. Look closly at the fuel system. Not pressure Volume. You can have plenty of pressure but not enough fuel getting to the carbs and it emptying the bowl,its gonna use a lot of fuel at WOT under the load of the car.

One other thing may be that the Air box isnt feeding air to the carb. Thats probably not it but worth looking at. We managed to make ours run truly bad and found out the tube we were feeding the air cleaner with was too small. NO VOLUME. There is a Theme here.

Last thing,Exhaust pinched off,too restrictive a muffler. Cant run to redline at the airport if we run the street muffler to satisfy the neighborhood. but knowing that we work around it.

Hope this helps.

Dan 77 IT7

[lateapex911]

To follow up what Dan says, make sure you've got a good fuel pump, a good regulator both that flow well. Check your float levels, and baffling.

[rx761]

I have a Holley red fuel pump with a Holley fuel pressure regulator. The fuel pump is less than a year old. Fuel pressure is set at 3 psi.

[dickita15]

I have a Holley red fuel pump with a Holley fuel pressure regulator. The fuel pump is less than a year old. Fuel pressure is set at 3 psi.

That’s all well and good but you can still have volume problems. We learned on the dyno that we were leaning out above 7K. the cure was larger fuel lines.

[rx761]

Ok. Guess it is back to the dyno then.

[lateapex911]

But, remember what Dan said about the track and thedyno not being the same. you'll get much more fuel slosh and foaming on the track than you will on the dyno. So, is there a glass window into the float bowl as there is on the stock Nikki? If so look at whats going on in the float bowl and see where the levels are.

Could be that the car is just foaming and shaking it up and it's running 'dry" at 7K, but will be fine on the dyno. So experiment with float level and baffling.