Mikunis IN/Strombergs OUT

[Harvey]

I offer this to those of you with Stromberg heads who have thought about a carburetor swap but who do not want to convert their heads to run Webers.

I have owned my Stromberg head '71 Plus 2S for 23 years. A few years ago, I began looking for ways to get more than 84 hp and 84 lb-ft of torque it recorded on a chassis dynamometer. Ken Gray at Dave Bean Engineering suggested boring and stoking the engine and fitting higher lift cams.

So I fit a Formula Ford 1600cc crank with a .060 inch overbore that increased the displacement to 1720cc. Next came a Dave Bean 114 intake cam and Sprint exhaust cam. The head was also skimmed, which upped the compression ratio to 10.2:1. These modifications yielded a noticeable performance increase, but, unfortunately, not the dramatic difference I had hoped for.

To ensure the Strombergs were performing at their best I decided to have them rebuilt. That is when Ken suggested I fit Mikuni flat slide carbs instead. I was resistant at first thinking this was one step too far in departing from the originality of the car. But then when I considered the multiple changes I have made to the car over my years of ownership - beginning with fitting a Spyder chassis - I thought one more change will hardly diminish its originality or character. My thinking was reinforced after speaking with another Plus 2 owner who had already made the Stromberg/Mikuni swap and reported a significant improvement in throttle response and power.

The company selling Mikuni conversions for British and Swedish cars is Vintage Performance Developments (VPD) in Syracuse, NY. The particular Mikuni recommended by VPD for engines under two liters is the HSR 42. The cost for a pair of these carbs (including choke assembly, collars for fitting the Lotus OEM air box, short idle adjustment screws and the jet tuning kit was less than $1,300.

Overall, the carbs were easy to install; however, the mounting bracket for the chokes needed to be modified to provide bonnet clearance because my car runs a Euro intake manifold with balance pipe. This manifold causes the carbs to sit higher than they would with the U.S. emissions manifold. The Lotus owner I spoke to running the emissions manifold had no clearance issues using the bracket supplied.

When I was able to road test the car, I was thrilled at the difference in response the Mikunis provide when you pull those slides open. VPD claims the HSR 42s flow 25% more air than the Strombergs.

To ensure that the engine air/fuel ratio was correct with the Mikuni setup, I decided to book another chassis dyno session. The dyno operator enriched the idle mixture and noted that the ignition was breaking up at higher rpms. He replaced the distributor cap, plug wires, and coil to cure that problem. This proves the old saw that half of "carburetor" problems are likely to be actually ignition problems. The dyno operator thought the jets and needle position chosen by VPD were close to spot-on and ideal for a street car.

At the end of the dyno session, the engine made 104 hp at 6,000 rpm, 97 lb-ft of torque at 4,500 rpm, and produced a torque curve that was close to flat from 3,000 to 6,000 rpm. Although my engine is no longer stock, comparing these results with a stock Lotus big-valve twin cam running Weber carbs, which Rohan has said will make 90-95 hp on a chassis dyno, I was very pleased with the results.

While my almost 25% gain in hp since my last dyno session cannot be attributed solely to the Mikunis, I can say that they made the biggest difference in "seat-of-the-pants" performance increase of all the modifications I made to the engine since then. Perhaps the additional air flow the Mikunis provide was needed to take advantage of the engine modifications I had made earlier.

Lee

[miked]

Hi Lee,
Sounds very interesting. Some photos would be great. Thanks for posting. I did see an article several years ago about a guy who fitted a down draft weber and developed a suitable manifold for a Europa twincam. It is good that there are options for Stromberg headed engines.
Not familiar with your carbs but Japanese carburettors have had a lot of development. I run japanese bike carbs on my Zetec. Nice and clean burn. Good starter. No dibbles.
You probably know the Lotus police will not approve.
Mike

[seniorchristo]

you tube video on the subject

https://www.youtube.com/watch?v=NpcQkC_o76w

[Harvey]

Mike,

Thanks for your positive response. I am attaching a few pictures I should have included with my original post.

Lee

[The Veg]

Very cool! I can see my self doing this at some point.

[Glenn M]

Very interested in the setup.
Any pictures of the kit before installation.
How has the drivability and mileage changed over the ZS?

Cheers,
Glenn

[69S4]

How do they work with the airbox?

[Slowtus]

Hi Lee,
You probably know the Lotus police will not approve.
Mike

If they had seen some of what I had done to my various Lotus over the years - and its a shame the S2000 ensemble is too big and heavy for yer average Lotus otherwise...instant upgrade.

[Harvey]

Stuart and Glenn,

The Mikunis work just fine with the airbox. The adaptors sold by the vendor interface between the carbs and the airbox.

I haven't had a chance to check gas mileage since fitting the Mikunis but I'm sure its less than the Strombergs. With the extra performance and responsiveness, you won't care about gas mileage. The accelerator pump on the Mikunis makes the engine much easier to start. One prod of the throttle and the car springs to life.

I'm in the midst of changing instrument panels at the moment but am excited about the thought of driving the car again soon. The carbs have transformed the car for me.

Lee

[Harvey]

Glenn,

I forgot to add that I don't have a picture of the kit before installation.

Lee

[The Veg]

With the extra performance and responsiveness, you won't care about gas mileage.

So are you saying that you'd agree with the "YEE GODS!" testimonial on the supplier's website?

[Harvey]

Veg,

Couldn't have said it better myself.

Lee

[jayrz]

Well mine are now on their way from VPD. Mikuni HSR42 set.

Vapour Lock bit me Sunday morning after a 40 minute drive to the gathering point for a local lotus club drive... Car wouldn't start and drive commenced, 40 minutes later got her started but group was long gone obviously,,,that was the last straw for me.

Jay

[arcangel]

VPD had some delivery issues in the past. This is the only reason I'm holding back on ordering my Mikuni kit. I'm very much looking to see if they are past that and you get your Mikuni's in a timely manner.

[jayrz]

Arcangel,

I'll update as soon as they arrive, hopefully not an outrageous wait but i had to "pull the trigger" because I've just had it with these Strombergs.

In other news Sunday i ordered some items from Burton Power in the UK, that was Sunday afternoon. A 3.54 CWP set, Hi Spec (UL4) 4 piston calipers with Mintex pads, needed hoses and banjo bolts. The order was processed and shipped Monday morning and in my hand here in Denver Colorado last night, incredible.

Jay