Mikunis IN/Strombergs OUT
Posted: Thu Apr 14, 2022 1:52 amI 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
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
Good starter. No dibbles.
